Hahahaha - Mike asked for the "best way" so I thought I'd offer my opinion of these suggestions.
Kraig's method - not really the "best way". Probably end up with a measurement like 40 which would be a real stumper.
Also didn't mention whether he meant measuring from inside or outside the runner and the closed end. Pics were nice. Overall the post probably just adds to his post count.
Kendal's method - doesn't involve measuring so can't say it's the "best way" to measure, but sure fire way to know which deck you have and the size if it does have a serial number tag. (I've seen so many that didn't have tags, and didn't have signs of ever having a tag - could never figure that out).
Jerry's method - well, could be reasonable method but don't know if Mike has a septic.
Digger's method - most likely the "best way" but needs some clarification. Said to "flip it upside down". Best to remove deck from the tractor before you do this. Also said "measure blade tip to blade tip". Sure he meant this to mean the outer 2 blades. So overall, this method requires removing the deck from the tractor, flip it upside down, rotate the 2 outer blades so they are aligned tip to tip, and then measure from the outer tip of one blade to the outer tip of the other blade.
P.S. - if you're deck doesn't have blades installed, and doesn't have a serial number tag, well.... you're still not screwed. You could post some pics of the deck. Also measure across the centers of the 2 outer spindles and maybe someone else on here would check that measurement against their own deck that looks like yours.