There is, its called a clamp meter and digital multimeter.
Once you clamp the amp you will know the power the amp is making.
There is now way to really tell other then that due to a speaker being a variable impedance, and with every frequency the speaker plays will result in a different amount of power being giving to the speaker. A
Another words if you are 100% sure the amp is making 80watts rms at 60hz, it does not mean it is making 80wrms at 100hz, or 50hz.
I wish at least the power at xx hz is consistent lol, or that they made something like a chart or something on amps and or subs that say...at this hz, this amp will make this power, and this sub will handle this power, and have it written in a way that everything just stops at a volume level of less than 1% thd and not be able to go any higher. Of course, i know that then again, all of those will vary dependant on different voltages, but i think there should be stricter regulations for specifications and ratings.