Leak down test and run dye through your coolant. Compression test will also help. The crummy ford coolant (**** yellow stuff) has the propensity to evaporate (so I've heard from a friend who is a Ford Master Mechanic). He recommended water wetter and/or royal purple w/Peak. Apparently keeps the block nice and cool. If you blew a head gasket you'd know (steam through the exhaust, rough idle, loss of power). At least I hope it isn't a blown head gasket. If it is Gil, you'll have some work to do.