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.

Just a simple audible comparison of one speaker and then the other can be very helpful. In many instances both speakers are not damaged at the same time and, if they are, it still may manifest itself differently in the two speakers. A CD or LP of test tones, such as the Stereophile CDs or the Realistic/Stereo Review test records can be used with a db meter, available from the local Radio Shack. As stated above, speakers can have pretty ragged responses in-room, however if the two speakers are placed in exactly the same position with the mic of the db meter in the same place and there are marked differences in the responses of the speakers then a one-way trip to Goodwill is called for.

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.