what with the relatively small cost of a decent quality Shimano BB. this is one instance, where i take the bike to the LBS, play dumb (or am just objective ) , and let the mechanic tell me what i need. then i buy it from them. if they don't have it i will usually order it from them while there.

in the real world, i have enough spare square taper BBs on the shelf or on bikes that can easily be removed to just try one and make a reasonable guess as to how long it needs to be. then i'll order it online and have it within a week.