It's easiest to measure if you have it out of the shell, or at least have the rings off.
If it's a cup & cone type, you might be able to overhaul it with new BB's & grease. I can get new BB's at my LBS for $.05 each, so $1.10 to do both sides.
IF the spindle or cups are pitted, it would be a good time to upgrade to a cartridge. Replacement parts, IF available, cost too much to be economically feasible.
IF you have an older C&C type, you have to note if it is built with an offset to one side. Both my MB's were built with the drive side 3MM longer than the NDS. Since cartridge types only come in a "symmetrical" style, the replacement cartridge had to be an additional 3MM longer than the existing spindle. That moves the NDS crank out 3MM. See-
http://sheldonbrown.com/harris/botto...ets.html#axles