There really aren't a lot. When they spec it, just make sure they also tell you the tolerance (+/- x db). Some speaker companies will do specs like 40-20kHz +/-1 on axis, 30-20kHz +/-3db. This gives you a little more sense of where most of the variation is.
In general, look for quality drivers, solid cabinets, etc.
In general, look for quality drivers, solid cabinets, etc.