I'm not sure the intake manifold gasket will allow water into the sump. I guess if the valley gasket fails or the manifold is very loose, water could get into the vee.
A cylinder leakage test will tell you if you have head gasket failure, and where it is. Leakage testers are fairly cheap at Horrible Freight and I find this an essential tool for troubleshooting engine problems; moreso than a compression tester.
A cooling system pressure test will tell you if you have a leak somewhere in the system, but will not pinpoint it's location.
All that said, if you have chocolate mousse in the sump, that is not a good sign and it probably does indicate a head gasket leak.