Usually what I quickly do is look at the stripes: if I can clearly see white stripes all the way down the body, it's Zebrasoma velifer. If they fade halfway down the body, it's Z. desjardinii. That doesn't help much here, but another thing if you're up close is to look for yellow spots on the face (as here) which you can find in Z. desjardinii. Spots on the tail and brighter yellow stripes also help point out a Z. desjardinii for me. Just my quick tricks, but I'm interested to see what others look at.
