According to a leaked file from Guantanamo Bay, interrogators were told to look out for people wearing the Casio F-91W digital watch because that was standard issue for Al-Qaeda bombers-in-training.

The government found that more than one out of three Guantanamo prisoners who had those watches, quote, "have known connections to explosives."

(BBC)

Here's a couple of pictures of the Casio F-91 W, so you know what to watch for: