When the girl is emotional. There are a couple of things I like to do.
1. If I've got a decent amount of rapport with a girl (she's paying attention to you, smiling, and receptive to kino), I like to do a quick bathroom takeaway. Me: "Hey, be right back." And just peck her on the lips. Not full-on makeout; just a quick peck. And then I take off and come back. She's usually down to make out after that, because the barrier's already been broken.
2. If a girl is getting really high-energy and silly (joking around, acting crazy, saying nutty illogical stuff), I'll keep the jokey fun vibe going by telling her to stick out her tongue. And then licking it. Pull back, tell the girl she loved it, make out. In fact, this is a great response to congruence tests.
3. If a girl starts talking about something and gets herself really emotional (doesn't even matter what the emotion is... as long as it isn't anger or disgust, I guess), I say "hey, c'mere" and make out with her.
Don't be afraid to go for the makeout if she's receptive to kino. You'll get the cheek, but that just means that she's enjoying the tension. She's getting validation out of the chase, and she wants more before she'll "give in."
MAKE SURE YOU UNDERSTAND THAT.
Getting cheeked does not mean that you "failed." It means that the girl isn't ready to make out yet. You're supposed to get the cheek. This is a congruence test. She wants to see if you'll maintain your state or wuss out afterwards. Just act like it's fine that you got the cheek, keep the interaction going, and go for it again later.
Or it could just mean that you're not sufficiently isolated and she just doesn't want anyone to see her kissing some guy she just met. Isolate the girl and you've got yourself a makeout.