Id the crank, then look up the manufactures specification?

For the 1.37 x 24t, that's the thread spec, but in bike terms, it known as an English thread, You won't hear it talked about in day to day usage as a 1.37 x24t unless you frame building/designing/manufacturing BB's.

The simplest thing if looking at the crank is to get an external cup BB (i.e. HT2, MegaExo or GXP), then you don't even need to think about the spindle length, as it's fixed.