easiest thing to do is read the label on it. It probably has two 12V rails, so just add up the number of Amps for both.
Except it doesn't work that way.
You can't just add the two (or more) numbers together to get the total output. Take my Delta based Earthwatts 500W PSU for example. "Two" 12v rails, each rated at 22A. According to you I should have 44A but I don't. There is a little note right beneath the PSU chart that says 12v1 and 12v2 can only output 408W at the same time. This comes out to 34A. Each rail by itself might be 22A, but the source feeding them both is only capable of sending 34A total. Another way to show thats wrong is that 44A x 12V = 528W, which is more then a 500W PSU can do.
The PSU you linked isn't any better then the ones he's suggested already. Has the red voltage selector switch, so its based on an old design. This is farther backed up by the use of an 80mm fan and 75% efficiency. It also has to much power available on the 3.3 and 5v rails, and no info on how much power is on the 12v1 and 12v2 rails total. I wouldn't use that PSU. (
Emphasis added by Gupeez).