Twexcom, are you asking how to measure the max wattage of a speaker? I don't think that you could do that, outside of putting too much through one - "I guess that was the limit."
I've read some posts here that stated that the max rated wattage for a speaker didn't matter all that much, as long as your amp (or receiver) had enough power to not clip. I'll let the speaker gurus chime in on this one, though.