As a consumer, there's very little you can do unless you get the services of thrid-party audio labs used by hardware reviewers and have your amp tested in terms of current and votlage measurements on specific loads. Then the power can be derived from such measurements.
Because of this, I often have to rely on the honesty and prudence of manufacturers to state the power and other technical specification of their product conservatively. That is why I put my trust on certain brands over others, often at the risk of sounding racial, since some brands from some countries are known to hype their specs more so than others.
Often in our rush to buy a reciever or amp, we rarely inpsect the back panel which can reveal a lot of information. I usually check out the electric power consumption stated at the back to make sure the claimed power output is within the electric power consumption. You can't give out more than what you took in. Also, you have to consider that all amps are inefficient and give off heat instead of turning all their work into useful sound. Often, depending on the design, the electric power comsumption has to take care about half of the work done to give off heat leaving only a good half or less to account for the useful power output. You should start to suspect any claims of say, 100wpc on a 5.1 reciever or amp, if the electric power consumed is less than the arithmetic sum of the power per channel figures. Then there's speaker impedance minimum rating. I wouldn't get any receiver or amp with a minimum rating of only 8-16 ohms. That's indicative of the amp's limitation in handling low impedance loads that can come with certain frequencies in the audible spectra.
As a consumer, I would suggest that prior to getting an amp, you owe it to yourself to read as much as you can from internet or magazine hardware reviews that have a more objective or dispassionate view of the product they are testing to verify manufacturer's claims. That, plus having a thorough audition can help you in your decision.
