My tetras totally lost color during the stress of moving from my LFS to the tank. They looked silver/gray until they were acclimated and released into the tank. When they were comfortable, they colored up to bright, vibrant colors
For me, I've just gotten to know my fishes' personalities and can detect subtle changes. Most of the time, as others have stated, it's a loss of color, timid behavior, hiding, sometimes they'll get spooked easily, and a loss of appetite. Those are usually the biggest red flags.