There should be a label on the side of the PSU that gives specifications. It should list amperage capacity on the 3.3v, 5V, and 12V rails and total watts the PSU can provide. Here's a great primer on PSUs at JohnnyGuru: http://www.jonnyguru.com/forums/showthread.php?t=1036
Please see my signature for a THG PSU thread and recommended PSU thread.
We can provide you with more information than the label, though. The label won't tell you if your PSU's from a brand that's considered reliable or one that'll burn before coming close to its rated wattage.