Personally, what I find is that I tend to be friends with people OC first - so know how they are with flirting - and then that leads to flirting IC. But at the end of the day, just flirt as you would OC, and use normal visual cues to see if they are not interested.
If your character has a crush on their character, I reccommend speaking to them OC if you plan on trying to persue a relationship, but like I said before, flirting with someone IC is just like flirting OC, if someone is not into it you can tell, and you stop. And they know its within their right to break character to say “can you stop?”, but yeah … just use normal visual cues. If they look uncomfortable stop, if you think they might be uncomfortable, break character to double check that it might be IC uncomfortable or OC uncomfortable.
Generally, as with real life, its all about communication and body language cues.
Good luck! (And feel free to flirt with my character Sneery of the Wyverns Watch in Wintermark - I tend to find it hilarious and depending how drunk Sneery is, Sneery tends to find it good fun too!)