Usually the rated amperage is stamped on one housing half, along with the voltage and part number. Do you know what series the Alt is? ie. 10si, 21si, etc. How many wires does it have coming out? (or terminals) The small frame alts range from 37amps (standard for older cars w/o power acc) to about 70+amps (newer with power acc) and newer frames even higher.Larger frame alts go up to about 130amp or so.