I find the best way is to look at them briefly then decide. Obviously, find smallest and largest first. Sometmes the answer will be very apparent, but usually you must then decide between the 2nd & 3rd biggest.
I have found the longer I stare at the angles the more they begin to look the same. Try a practice test by moving quickly through them and going with your gut instinct. I bet you'll find that you will do better than you think.