Loads of extras on their profile, pickiness about their clients, insistence on being classy courtesans who like to be wined and dined instead of just being an honest cum slut. Those are all hints.
On the phone if they're just a little bit more difficult to book than is reasonable, and not because they have lots of bookings.
Prices that move up and down far too much or change after a booking, messages about `letting them selves go too cheap', it all tells you what their thinking is, i.e. probably not as obsessed with bouncing up and down on your cock, licking your bum hole and then taking a facial as you want them to be.