Is there any way I can bench test speakers?

Get a test record/CD with a frequency sweep, and play each speaker at various volumes and see if they break-up, buzz, crack, or do anything that sounds bad. If they don't then do it in front of your buyer so that he is satisfied of their soundness. Then if he blows them up, you are off the hook.

Twl is right on. A Db meter can help. Bear in mind that unless you are in an anechoic chamber all measurements will be askew.

Sure, but you're gonna need at least 2 spotters, given the assymetric nature of most speakers, let alone nothing firm to hold on to. To each his own, but personally I'd just stick to a barbell and plates. Much easier, and probably safer.