How can i find the actual Watt rating for a Memphis Audio Amplifier?

I received a Memphis Audio amp from a friend and he says it is a 500 Watt 2 channel amp. I need to know how i can figure out if it actually is 500 watts, i have no reason not to believe him, but the amp does not say its wattage on the amp's body which i think is strange. It has two "15" Fuses in it if that helps.... it is a solid black 2 channel amp with a memphis audio "M" that illuminates blue when it receives power... it pushes a 300 watt RMS rated sony Xplod no problem... it is about a foot long and 6 or 7 inches wide... and about 3 or 4 inches tall... any info on this amp would be appreciated