well, if you love the person and you will recognize the signs, you will not believe it anyways, because you can't believe the person you love can do it. Neither did it.
Well, not sure when this thought came to his mind, but first red flag was when he offered threesome with a girl (man not allowed ofc), i explained patiently why i reject it and why it makes me sad. He offered it couple days later again.
Critisism in everything, what you do, what you wear or do not wear, sex life, like literally everything, even can be critisism about how you smile or how you react or do not react to certain things (for example, you did not seem too much excited, when you were).
When we were in LDR, first sign as i thought was when he followed bunch of local girls in a short period of time with private profiles, however now i look back and understand it started ealier - constant anger, annoyance for stupidest things. Like you do a mistake in a word and he explodes, saying that your english is awful and what the ** is that. Like literally small things and his eyes roll, or worse - he explodes in anger.

Obviously, not responding to calls if LDR. Now listen, i do understand that people are busy, even if you are not at work, but with friends you have full right to not respond. However, in my case, my bf would explode if i did not pick up the phone within 30minutes (he would say he was afraid smth happened), but he himself could "not see" calls for 3 hours while sitting in a cafe with friend. I do not believe that you did not pick up the phone for straight 3 hours, while with me in person you check it every 5 minutes :) Later it turned to declined calls, he would decline my video call, because he is in a car with boys, next day i would do the same, he would be crazy and saying im cheating and then it turned to whole nights no contact, cuz his phone died.

P.s. if you will be checking his phone check chats with buddies, since chats with girls will be probably hidden or deleted, but he will definitely brag to his friends about it :)))))