Plus one to Gary. "Not reef safe" means dozens of different things to different people, and there is always variability from one fish to another.
Compatibility with broad categories of typical livestock is probably better though not exactly accurate in every case. Does the fish eat other fish? Cleanup crew? Stony corals? Softies? Clams? And so on. If you care about any of these types of livestock, focus on compatibility with it. Also, try to focus on a specific fish species, not a broad family.
Examples: Butterflies are sometimes considered not reef safe but specific species, ie copperbands, are generally fine except for specific conflicts that most people won't care about (they will probably eat any tiny featherdusters you have). Triggers are another good example. Several species will likely be fine (nigers, blue throats, sargassums, etc) though they'll eat snails so you'll need to restock your cleanup crew from time to time.
So decide what types of reef life you don't want eaten in your tank, then research your target species based on compatibility with that stuff.