Bleaching isn't really the loss of tissue, the coral just loses color due to expulsion of zooxanthellae and the tissue is still intact. However RTN (rapid tissue necrosis) is the degradation and loss of tissue due to a number of factors (of which we mostly don't have a clue
).
Usually you can tell by closely looking at the corallites of the coral, no polyps usually means RTN (for me anyways). Corals usually don't bleach overnight either they take a few days.