You can't, you'd have to use a Kill a watt. I'm thinking he meant the bios will tell you what the voltages are coming into the motherboard. Under a "PC health" section it should say something about the 3.3, 5, and 12v "rails" of your PSU. When he said "under load of coarse it will use more power" that means the rails will go down a bit. So if you look and your 12V rail in the bios says 11.5V, then you can expect "more" to be drawn when playing a game and it could go down to 11.2 or something like that. That is out of spec for the 12V rail, and your video card or CPU might have issues running with that little power being delivered.