I'm definitely not an expert on the subject, but you'll want to look out for a few things.
Eye contact, making excuses to spend lots of time with you, affectionate touch, and compliments are some of the big ones. Friends do some of those things too, but they're probably flirting if the seem to focus excessively on you, or if they get flustered---but if they're confident they might not.
It's the furthest thing from an exact science that I know of, that's why I'm so bad at it.
[text]
Eye contact, making excuses to spend lots of time with you, affectionate touch, and compliments are some of the big ones. Friends do some of those things too, but they're probably flirting if the seem to focus excessively on you, or if they get flustered---but if they're confident they might not.
It's the furthest thing from an exact science that I know of, that's why I'm so bad at it.