96 explorer coolant issue

Had overheating issue with the wife (she doesn’t monitor fluid levels like she should). Now you can’t operate it for more than a day or two before the reservoir is super low to empty. Mechanic coolant is going into the engine, probably through the intake manifold. Says investigating further will cost a ton. What can I do cheaply to figure out what’s going on before I give up? I followed her when the engine was started from cold & could see water vapor (more than I would expect) in the exhaust until when I figure the engine reached full operating temp.
Can y’all help me figure out what’s going on before I waste a good vehicle?
It has about 225k and is the 6 cyl engine, btw.

The first thing you’ll need is a cooling system pressure tester with the proper adapter. http://www.sears.com/craftsman-cooling-system-pressure-tester/p-00946342000P?prdNo=1 Some parts stores will rent or let you borrow one of these. Then pressurize the cooling system to see if it holds pressure. If not, and there’s no apparent external leaks then the coolant must be leaking into the engine. This could caused from a leaking intake manifold gasket or a leaking head gasket. In either case it still requires that the engine be disassembled to determine what’s leaking. If it’s found that an intake manifold gasket is leaking then those are replaced. If it’s found that the intake manifold gaskets aren’t leaking then the leak must be from a head gasket. And if that’s the case, the intake manifold has to come off in order to replace the head gaskets. So the intake manifold gaskets get replaced anyway.