one of the biggest tells is if it has rounded corners. Another way is to look at the page source and if it is predominantly made with tables, it's subSilver2 based, but if it's predominantly made of <div> tags using CSS, it's most likely proSilver based.
Those are the two main styles (the only two official ones from phpBB.com). Of course other custom styles can be made, but the vast majority are based off of one of those two.
