Might be a head gasket, but the upper and lower intake is dry, so probably not that. If the engine is running OK and not blowing white smoke from the exhaust when the cooling system is hot, then the head gasket hasn't failed into a cylinder bore. If a head gasket fails into a cylinder, it will show up on a cylinder compression test.
There are cooling system leak detection kits, often the auto parts chain stores will rent them. They include some fluorescent dye that's added to the cooing system and a black light used to look for traces of leaking dye.
You could also try renting a cooling system pressure check tool, which is a small hand pump and gauge that screws onto the pressure cap of the coolant recovery reservoir. You pump up the cooling system and see if it holds pressure. Once it's under pressure, you could use some dish washing detergent mixed in water in a spray bottle as a leak detector, the soap in the water will bubble in the presence of a leak.