Some power supplies have a little information on the back, probably not enough to figure find what you need. Most, if not all, PSU's have a big sticker on the side. You can find it when you open your PC case up and take a glance. The information is located on a power supply like thus:
Depending on the design of the case, you might have to physically remove the PSU from the case. Some cases have a carriage, others have the PSU upside down or whatever.
Some stickers look like this power supply rated at 220W:
This particular model is a 300W unit:
Most stickers are broken down on how much power is pumped through each rail. A few show general information and a few show a per connector basis.
Please note, that despite a PSU stating they're 300W or whatever, There's a lot of variations in the manufacturing process and quality control. I've seen 250W PSU's from a top quality manufacturer run the same equipment more reliably than a 300W PSU from a ---smurfy--- company. Most people usually don't have to worry since they'll rarely hit the upper limits of their PSU.