Opening up the case should be sufficient. The PSU in most PC cases is in the top back of the case. Its wattage output should be clearly labeled. If it's a good or even decent model, it should have amperage ratings on each output (12v rail, etc.) However, the PC PSU market is very crowded, and awash in absolute shit. Check what brand abd model PSU you have, post it here, and I might be able to give you some feedback on it. If you're going upgrade, there is only one brand to consider: PC Power and cooling. No fugly LEDs or transparent cases, and the just about the best design and construction of any PSU on the market. Miraculously, they're made in the USA.
After an untimely power failure in 2006, I now use nothing but PCP&C PSUs (and APC UPSes.)