There are books out there that will give guidelines, however, it was experience that taught me. It will take more time in commercial than residential usually. What kind of fan motor, Shaft condition, bearings, single phase or three phase, why did it fail.
“Now the freaks are on television, the freaks are in the movies. And it’s no longer the sideshow, it’s the whole show. The colorful circus and the clowns and the elephants, for all intents and purposes, are gone, and we’re dealing only with the freaks.” - Jonathan Winters