It's often listed on the label of the PSU, or in the specs.
Here is the Corsair:
See how under the three 12V rails (18A each), it stated 480W. That's the total load across the 12V rail. Since 480/12 = 40, that's 40A
Also, you often know immediately that a PSU cannot deliver the full sum of the 12V rails. Again, the Corsair: 3 x 18 = 54A. 54 x 12 = 648W. Howevr, the PSU itself is only 520W