Check with an automotive machine shop. There is a setup that can be used to pressure test a block while it is disassembled. Not all shops will have it. One of the guys that helped develop it lived down the street from me when I was a kid, so it's been around for a good while.

Look in the lifter valley and along the sides of the block in motor mount area and all along the line formed by the so-called "freeze plugs".