i would recommend what BUP stated, fill the manifold with piping hot water and pressure test or the chemicla test with heat if it is a hair line crack it won't show until it is hot from the exhaust going through the manifold. i did this with a gil mainfold and found a hair line crack right on the manifold cylinder exhaust port splitter.

Well tried the old acetone trick on my emi...Its been sitting in the manifold for a couple hours now. But I noticed that the level in the tube went down about 1/2 inch. However there is no water in the exhaust ports.. Evaporation??