is there a way to tell if someone is wearing a microphone without stripping them down

I don't think so

There isn't always a wire, these days they have little beltpacks like wireless mikes. Sometimes they'll just use a narc radio. Narcs are good at concealing all the microphone wires, wireless earpiece transmitter, pistol, ... especially on surveillance jobs.

Isn't their a way to make a feed back loop happen?

Wear one of those wire scramblers.

I saw it on The Shield.

You could just ask them if they were wearing a wire.

On some TV program I saw, they used a Handymans 'Nail finder/ stud finder'

Its a tool used to find nails in walls, so you know where the stud is.
Works on magnetics I guess.

This is why you should do all of your deals completely naked. Everyone.

They talk to their shoulder...

I remember on attack of the show...they put a wireless server in a guys ass.

and a girl might just say that string hanging out is her tampon cord