I should hope that's already being done --- all phone lines to a mosque should be wiretapped and everyone entering and leaving the mosque should be checked out. How much time they spent in there should be noted, any new muslims showing up should be noted, passports and everything checked out.
Do yourself a favor and Google your town or county +islamic and watch how many Islamic centers/mosques come up. I did it for conservative Orange County, CA and got some 7 different places for worship/study. Scary, but at least I know where they are now. Yes, Islamists, I'm watching...so what?