Are you talking pre-snap? Post snap it’s easy to tell as your safeties won’t play deep hash in a Cover 2, it’ll be combo coverage. Pre-snap, there are a couple of triggers but the easiest way to see it without having the power of hindsight and coaches tape is by catching the positioning of the CBs and the MIKE. That will give it away. If the MIKE is playing the hook and/or you’ve got a press man or bump off look outside, you’re playing quarters. This is what we’re predominantly showing now.
Not a dumb question either, very good question. The defense is designed to force the QB to make his read post snap which is never in his favor, especially at the college level where one read offense’s are the norm.