How manywatts are going to my PA speakers??

Ok, ive read my instruction guides to the items of my PA and they just dont seem to much help sometimes. I Have a 750 watt per channel Poweramp. and it is hooked up to 2 15 inch 500 watt program Yamaha speakers. On the power amp there are 2 knobs, and I dont know how much Wattage i am sending to my speakers when i turn them, I dont want to blow my speakers so how do i know how much i am sending to them??????