You can bench test the amplifier yourself with an oscilloscope, a volt meter, a clamp meter, a static load, a signal source, and a sine wave recorded onto media (i.e. CD). However, to actually buy all that stuff is fairly expensive, so if you really want to know, your best bet is to take it to a shop and have them bench it for you.
Of course, this information is almost completely useless as your speaker does not present a static load to the amplifier, so the potential maximum clean power output will change w/ frequency ... If you have a decent amplifier brand and set the gain using your ears or using a digital multimeter (or an oscilloscope, if you have access to one), you'll be fine.
BTW, what the hell were you thinking when you made this in the off-topic section of the forum instead of the amplifier section or general car audio discussion ?