So if I have a square taper bottom bracket spindle and I want to know if it fits a 68mm or 70mm bottom bracket shell...how do I know? How can or where do I measure the spindle? Thank you.

You measure the assembled bottom bracket. Are there no markings on the axle that might give a clue?

It is a Sugino bottom bracket. It says 3NB and 81-L as best I can tell. I guess I was looking for guidance that says "measure from here to here" but 2mm does not leave much room for error.

The "3" means a 68mm shell width (and a "5" is a 70mm shell). The "NL is the axle's lengths beyond the bearing tracks. The "81-1" is likely a date code. Andy

Thank you so much Andy. You are always a wealth of information.