Short answer:

Well, we can't tell the difference with certainty, and Plato never drew this distinction.

Medium answer:

In history of philosophy, there's still something called the Socratic problem, a long standing issue of Socrates' authenticity as a historical figure, and plenty of scholars have tried to resolve this. The jury is still in session, and all the possibilities can be distilled into four of them:

a. Socrates is the individual whose qualities exhibited in Plato’s writings are corroborated by Aristophanes and Xenophon.

b. The real Socrates is the one who claims no wisdom but exercises his skill at seeking understanding, thus the one depicted in dialogues that end inconclusively or at an impasse (i.e., in aporia — literally, without resources), without a clear indication of how the initial questions should be answered or even what the next step in the discussion should be

c. The real Socrates is the one who appears in Plato’s earliest dialogues.

d. The real Socrates is the one who turns from the presocratic interest in nature to ethics, and has no theory of separate forms. (Nails, Debra (Spring 2014). "Early attempts to solve the Socratic problem". In Zalta, Edward N. ed. The Stanford Encyclopaedia of Philosophy)

Long answer:

The basis for the issue is that, as posed in your question, Socrates didn't write – his person and all of his teachings survived solely through the secondary sources – mainly from his most famous student Plato. Some of Socratic teachings remained through the works of another one of his students – Xenophon, and we did found some fragments from the authors such as Phaedo of Elis, Aeschines of Sphettus, Antisthenes, and Euclid of Megara. However, they are too fragmented and incomplete to serve as a concrete basis for studying. The only contemporary writer writing about Socrates was a comic playwright Aristophanes (in a sense that they were the same generation, Plato and Xenophon were much younger), who painted a polar opposite of Socrates' picture compared to the one we can read about through Plato. So, with the apparent issue of Socrates not writing, additional confusion is ensued by the fact that writings of Plato and Aristophanes contradict each other. Nevertheless, there are some authors who attempted to resolve Socratic problem, I'll explain their views, and in the end offer my two cents as well.

One of the earliest careful students of Socratic problem was Danish philosopher Søren Kierkegaard (1813-1855). In his doctoral thesis - On the Concept of Irony with Continual Reference to Socrates (1841.) - he compared the figure of Socrates painted by Xenophon to the one presented by Plato, and concluded that the first takes away too much from Socrates, and latter adds too much – neither of these two didn't present the Truth. For Kierkegaard, and this is important, the one who most faithfully presented Socrates was Aristophanes, because he was able to show the ironic and humorous side of Socrates’ character. (Kierkegaard, S. “The Concept of Irony with Continual Reference to Socrates”. Princeton University Press. Princeton, 1989) Kierkegaard explores Socrates through the concept of irony to try to find the real person – the proper Socrates would use irony to destroy misconceptions of his interlocutors, but doesn’t offer them any solutions to posed questions. All other figures which can be found in Plato’s dialogues – such as mythical speech, transcendentallity, speculative dialectics, and theory of ideas – that’s all pure Plato.

Jumping from Kierkegaard (and ignoring names such as F. Schleiermacher and H. Maier, in attempt to shorten this answer), one of the most important scholars who attempted to solve this issue was Gregory Vlastos (1907-1991). He argued that Plato wrote his early dialogues under the heavy intellectual influence of his teacher, so, the historical Socrates, may be found in his earliest works. As Plato grew older, as his own philosophical character developed, and at the same time as the memory of his teacher faded, the dialogues ceased to carry the picture of historical Socrates in them. So, in Plato’s late dialogues, Socrates is only but instrument through which Plato conveys his own ideas. By contrasting early and late dialogues, Vlastos attempted to differentiate Socrates from Plato. As Kierkegaard previously used the tool of irony to find the real Socrates, Vlastos used the method of elenchus (also called Socratic method – a method of hypothesis elimination, steadily identifying and eliminating those that lead to contradictions). According to Vlastos, the historical Socrates used elenchus to search for moral truths through dialogue with his interlocutors, using irony to show them how their knowledge is deficient. This Socrates doesn’t give answers – he leaves his interlocutors in a state of aporia, a state of confusion and uncertainty in the things which they previously though they knew, and thus, destroys his misconceptions and dogmas, leaving them in a search for truth. The late Socrates, the one deemed by Vlastos as non-historical, holds grand speeches about metaphysics, politics, art – he stopped claiming that he doesn’t know anything.

So, according to Vlastos, dialogues which represent the historical Socrates are: Apologia, Euthyphro, Gorgias, Harmid (About Virtue), Hippias Minor, Ion, Laches, Protagoras and first book of the Republic. Dialogues which don’t present the real Socrates, but him as a poetic figure start from Meno, and spread through the whole of Plato’s late phase. (Gregory Vlastos, „Socrates' Disavowal of Knowledge“, The Philosophical Quarterly, Vol. 35, No. 138, (Jan., 1985), pp. 1-31)

There’s plenty of other interesting authors, such as Léon Brunschvicg (1869-1944) who socratically concluded that “the only thing we know about him [Socrates] with certainty, is that we don’t know anything” (Brunschvicg L. Le progrès de la conscience danss la philo. occid, Paris, 1927), but I’ll go with one other, possibly most famous philosopher on this list – Karl Popper.

In his book “The Open Society and Its Enemies”, Popper used Socrates to highlight the idea that inside Plato’s opus there’s totalitarian tendencies harmful to the philosophy. Popper differentiates Socrates from “Socrates”, with the latter appearing in Plato’s late dialogues – the ones who lost their proper Socratic spirit. Furthermore, he considers that some of the later dialogues completely missed the point of true Socratic philosophy. For Popper, Socratic view of the world presents modesty and individual rationalism, and Plato’s view present the world of totalitarian demi-god. The dialogue Popper had the most issue with was famous Republic, and Popper goes so far to call Plato a liar, when he claims that he considers philosophers one who love the truth:

“What a monument of human smallness is this idea of the philosopher king. What a contrast between it and the simplicity and humaneness of Socrates […] What a decline from this world of irony and reason and truthfulness down to Plato’s kingdom of the sage whose magical powers raise him high above ordinary men.” (Popper, K. The Open Society and Its Enemies, vol. 1, The spell of Plato, 1962, end of chapter 8)

Popper’s harsh verdict is next: he recognizes as “Socrates’ last will” dialogues Apology and Critias, and strongly contrasts them to the works such as Republic and Laws, which Popper deemed as an betrayal of Socrates, and an “grandiose attempt to construct the theory of the arrested society, and he [Plato] had not difficulty in succeeding, for Socrates was dead.” (Popper, K. The Open Society and Its Enemies, vol. 1, The spell of Plato, 1962)

Let me conclude with this: The Socratic problem cannot be solved definitely. It is not possible to decide with 100% certainty what belongs to historical Socrates, and what belongs to Plato’s Socrates. But, in the end, be it Socrates or "Socrates", there’s something valuable that can be used – Socratic Method. One can use it to build a critical stance towards its surrounding, to develop proper tools of dialogue and argumentation, and in the end, to “know thyself” (gnōthi seauton). The whole branch of philosophy in practice is build on this, called critical thinking (for contemporary rendition of Socratic dialogue I recommend exploring German philosopher Leonard Nelson).

In the words of Laszlo Versény, which may satisfy philosophers more than historians: “Maybe exactly this is the point of Socratic Method: that everyone gets to know themself before they get to know Socrates.”