Compression test is not always definitive for a leak, either. Does it overheat? Any evidence of bubbles in the radiator? There is a chemical test for head gasket leaks. Sometimes a tiny leak won't show up cold, either.
Run it awhile, on the road if possible, get it out and up to temp, then let it sit and then look at the plugs. The cylinder(s) that have a water leak (if any) will look "different," wet and grey.
