Hmm… Everyone’s different, of course, so I can’t tell you for
sure what behavior would be flirting. A lot of people used to think my politeness was a sign that I liked them, when it was really just me being polite. So depending on the situation, it can be hard to tell.
However, you specifically mentioned comments on appearance and personality. These comments
can just be polite compliments, depending on how they’re worded, but if it’s things like “you look beautiful,” “that’s a really pretty dress,” “that color looks really nice on you,” and so on, those are much more personal, and probably meant more affectionately than just a complimentary observation. For example, if I met someone who played the violin really well, I’d tell them they’re really good at the violin and usually something like I wish I knew how to play. Conversely, if my girlfriend was the one playing, I would phrase it more like “that was so beautiful, you always play so well, (when are you going to finally teach me how to play it

).” Subtle differences, but the intensity of the adjectives and their connotations are more personal when I’m talking to my girlfriend versus someone else who’s talented. I don’t even know if any of this is helpful to you… I also wouldn’t give a random person, or even an acquaintance those kinds of compliments out of nowhere, it’d be after a performance, whereas I’ll tell my girlfriend as I think about it.
I personally give people compliments if I feel they’re appropriate. I wouldn’t say I comment on their appearance, personality, talents etc. in general though. A compliment to cheer someone up, acknowledge an accomplishment, or give congratulations, sure, but if in your case, you feel like someone specific is flirting with you, they probably are.