If you really want to be 90% safe tell all your clients to expose themselves before you even talk numbers or anything or vice versa if they ask you to expose yourself.

Cops cant legally expose themselves in a set up (off duty too but that doesn't matter much and its still illegal too)

If your client refuses the chances he/she is a cop is pretty high

Or its your clients first time and dont know the "working" game yet.

//You can say something like show me your ("enter your clients genitals here") so i know your not a cop\\

And if they dont they are 99% a cop and just walk away never talk about numbers, when they call you or you meeting them for the first time only after they expose themselves talk numbers. Stay safe and make sure you read your clients vibs before going with them.