At a bar or any other social place, I see this pan out multiple times but still can't figure out the how part.

example: Girl at bar sitting by herself, guy approaches she gives IOI's, engages in conversation, spends about an hour and a half talking to the dude. Next thing she get's a txt or phone call (or pretend) and she has to leave. No bye, no common courtesy, just straight up get up and leave.

usually I laugh but it has got me curious and can't figure out how to spot these time wasters from get go

TIA