On the side of your PSU there should be a diagram labeling all of the rails..
Mine:
edit: misinterpreted your question.. you want to know the real amperage of your PSU's rails.. well, not much you can do but get an ampmeter and open it and measure. A theoretical 15A is a bit low, especially since your GPU is drawing from it.. you should look in to a PSU with at least 18A on the +12V.
Mine:
edit: misinterpreted your question.. you want to know the real amperage of your PSU's rails.. well, not much you can do but get an ampmeter and open it and measure. A theoretical 15A is a bit low, especially since your GPU is drawing from it.. you should look in to a PSU with at least 18A on the +12V.