The spec sheet of the PSU should say the amperage of the various rails.
For instance, on this XFX supply.http://www.newegg.com/Product/Product.aspx?Item=N82E16817207013&Tpk=xfx%20550W
The amperage's are listed as.
+3.3V@24A, +5V@24A, +12V@44A, -12V@0.5A, +5VSB@2.5A
So 44amps on the 12v rail.
You can also figure out how many Watts that is.
Voltage x Current (amps) = Power (Watts), so 12v x 44a = 528W on the 12v rail.