Do muslims have a tattoo or something?
Maybe a secret handshake?
There's a short Muslim credo that's taught from birth -- Muslims around the world would be able to say it in Arabic:
"Ilaha illa Allah. Muhammed rasul Allah." ("There is no god but Allah, Muhammad is the messenger of Allah")
The Christians executed in a relief agency in Pakistan last month were ID'd by the fact that they couldn't (or wouldn't) say it -- then they were shot in the head.