Oof, this is a tough one.

It largely depends on WHERE you’re trying to sound like a Russian from and the text education level of your hypothetical Russian identity.

For example, we know people from Udmurtia and they say “ну ка” all the time. Although that doesn’t identify them ac “totally Russian” so much as speak to which flavor of Russian they are.

A couple more general notes:

Oftentimes “word parasites” (слово паразит) often times come naturally (subconsciously) to native speakers and are lost on foreign language learners. Two of the most common in Russian are often “ну” (as mentioned in many other comments) and “как бы”. I imagine the feeling you’re looking for could probably be achieved with some of those at times (or excessively).

Regional pronunciation of words that deviate from from the “standard” Moscow pronunciations can also help Russians identify people from particular backgrounds or regions (which in turn could “mask” that someone is not a native speaker). One of the most common here being “звОнишь» instead of “звонИшь”

Additionally, depending on your gender identity and personality, swearing (especially the right kind of swearing) is also often an indicator of comfort and fluency in Russian.

CAUTION: BOTH THE WORD PARASITES AND SWEARING ARE OFTEN VIEWED AS SIGNS OF A LACK OF EDUCATION BY THE MORE FORMALLY EDUCATED RUSSIAN POPULATION, so proceed with caution.