You should find a sticker on the PSU that looks like the diagrams in this link http://www.corsair.com/products/vx/default.aspx
If you are looking at buying one online then reviews should be researched and a good place to look at the stickers/plates is Newwegg as they usually have a few pictures including one of the spec plate.
The formula you need for working out Amps is watts divided by Volts. So to find the Amps on a +12v rail that has say 300 Watts you divide 300 by 12 = 25.
When you have 2 or more rails that state they each can carry 18 amps for example this only means that each rail can carry 18 it doesn't mean there is 36 Amps. You still need ti find out what the +12 rail has in Watts and divide by 12.
It's generally labeled on the power supply itself. Example:
If anything, try to find out the model of your power supply and google/newegg it.
If the label doesn't give you a straight answer for the total 12v amperage, simply divide the total output wattage for the 12v amps and divide it by 12. On the label above, the total output wattage for the 12v amps is 672. 672 divided by 12 is 56. The power supply has 56 amps on the 12v rail.
That may not work. The label only reports what the vendor CLAIMS the PSU can produce, often under unrealistic conditions (e.g. at 25C, when a more likely temperature inside a working PC would be 35C-40C). If it is a Chokemax PSU, it may not be good for 75% of it: http://www.corsairmemory.com/cinema/movie.aspx?id=62274...
The best sites for competent technical reviews of PSUs include www.jonnyguru.com, www.hardwaresecrets.com, and www.hardocp.com. A good competent technical review includes load testing; in fact Gabriel Torres at hardwaresecrets often goes above what's on the label to see how much power a given PSU can actually produce. Oklahoma Wolf at jonnyguru also throws in a 10% load test to check for efficiency, since the 80plus testing starts at 20%, and SMPSs are even less efficient at super-low loads.