How to tell if a head can still be resurfaced?

[serviceguy]

is there a way to check how much material has been removed from a head during resurfacing? I read somewhere in here that the oil inlet galley in a 4g63 head is also an inspection port used to determine how much material is left on the gasket surface and that the Service Manual says so...somewhere. Well, it makes sense, doesn't it...unfortunately I could not find trace of this on my Service Manual (Mitsubishi 91 Eclipse) or the Service Manual CD. I know the max material that can be removed is .2 mm but how to tell when buying a used head? Thanks