Intake manifold won't seal - How do I check it ?

My machinist came over today and looked the situation over a little. He told me to try a .100" thick gasket if I could find one. If not he would look around for me. He said this would be the first and cheapest step. If this didn't work then he would possibly have do some machining work and that would be a lot more than a set of gaskets, which is what he is trying to help me avoid.