There is no such line. Normal conversation segues smoothly into flirting if mutual attraction is present. You can’t really point to any one verbal exchange as flirting and others as not. It’s the mutual attraction that determines whether flirting is taking place, not the actual content of the interaction.
What you want to be looking at for distinction is conversations in which one person is attracted and the other isn’t. In these sorts of interactions, the person that is attracted is going to be deploying tactics to try to get the other person to be attracted to them. The term we have for this is that you are hitting on them.
While there’s no line between normal communication and flirting in the case of mutual attraction, there is indeed a hard line between normal friendly conversation and hitting on them. If one party suddenly starts bringing up, with no prodding from the other person, content of a sexual or even overly-sensual nature, like asking a young girl in school whether she likes to “party” out of the blue, or bringing up things like alcohol consumption, then that’s hitting on them. It’s generally uncomfortable for the person getting hit on. If you’re a straight guy that’s ever been hit on by gay guys then you know the feeling.
If mutual attraction is present, then it doesn’t really matter what the topic of conversation is, attraction is going to get concentrated and intensified by anything either of you do. Verification of the mutual attraction is simple. You just comment on how interacting with them is making you feel, and state your desire for more interaction, and then you just go by their response. Because you’re not just gunning for sex, you’re looking to continue interacting with someone who makes you feel good. In all kinds of ways.