I've been thinking about this a lot.

At what point should you tell a girl she's pretty?

I've had girls approach me in parties and ask me who I fancy in the party. Sometimes I ask them to answer the same question and they say they fancy me. So is it okay to tell them at that point or should I keep them on their toes longer? And after I tell them, how can I still maintain my "I don't give a fuck" vibe.

Can someone just tell me the best way to go about it please?
My assumptions are that girls like to keep guessing and that the longer I wait the harder they want to hear it.

Thank you guys!