Coolant Loss?

My blazer has been misfiring for a while now. The other day I went to start it and it had no compression. After turning it over a bit I was able to start it. Since it was acting up I checked the fluids. I was losing coolant from an internal leak. Thinking it was possibly a blown headgasket I checked the compression and all the cylinders were good, and there was no coolant in my oil. I think it may be an intake manifold gasket but how would I know for sure?