Look at the safeties. This will tell you most of what you need to know. If one is higher than the other and one is walked into the box, it's 100% cover 1 or cover 3. If they're evenly spaced, we're gonna need to do some digging.
Know how defenses align to your formation. If people run man coverage, they're not going to be base aligning. Therefore, if you have trips, look for more DBs to line up on the 3-threat side of the field. Look for formations where the TE or the RB is the widest threat on the field. Trips TE is great for this. If a linebacker or safety comes to cover him, it's man coverage. CB? That's zone.
Look at the distance off the line of the outside CB. If they're playing ~10 yards off, straight up, hips not turned, you're looking at man. If they're playing a little deeper, with hips turned ever so slightly, they're probably in a deep blue.
Look over the slot CB. If you see a guy about 10 yards off the line right over him, then that slot CB is coming on a blitz.