It's no different then if a test kit is showing a problem in my opinion. If the test kit is reflecting a negative reading it means the fish have already been affected but you missed the visual cues the fish where giving among others signs.not sure I agree with the fish showing signs thing, if my fish acting weird, color bad, off feeding because of water quality then it is already having an effect on them. is this not what you want to avoid as some dmg has been done to one extent or another?
I look at my tanks everyday and quite often so I can tell even slight differences in water color and clarity among other things and the same goes for my fish. I notice every little difference in their behavior well before the more serious signs of bad color, off feeding or rapid breathing. I can tell by how they react to my approach their general activity level etc.