Sighs....
Look back to the original example.
I have had an experience. Is that experience a true reflection of reality? We could put on our trunks and arm bands and go and see if it was a lake. Easy Peasy Lemon Squeezy.
You have had an experience of God. Is that a true reflection of reality? We could .......
It's an interesting example, since as an hallucination (not an illusion), it can be checked empirically. In other words, as you say, you can drive in that direction to see if there really is a lake, or go up in a helicopter. In the case of illusions, it's more tricky, I think, since there is something real going on, but not exactly as per the illusion (for example, the bent stick in water).
But maybe both these examples are poor analogies to religious experience, since this claims a supernatural process or event, or whatever it's called.
I find experiences in Buddhism and other Eastern religions interesting, as there are plenty of mind-blowing experiences, which can be loosely called non-dualist, or non-ego, yet no supernatural origin is claimed for them. Well, maybe you could have a non-supernatural God, of some kind or other.
But the supernatural God cannot by definition be checked out empirically. This is a nonsense.