The quick and dirty way to determine if you have a long or short water pump is to try and stick your fingers behind the timing cover, you can not with the short pump.
A more accurate way is to measure the shaft, a SBC long water pump shaft is 7″ and a short is 5 5/8″ — a BBC long water pump shaft is 7 1/4″ and a short is 5 3/4″.
