Jhanas change your existence, it does not work like hallucination or as just a simple state of mind. You can not pass experience in words (especially arupa jhanas); one needs to experience it and know by experience. For me, it’s like asking if you take something under a microscope and say, “How do you convince that you are not seeing a hallucination?”. It’s good to be skeptical, especially if you lack your own insight/experience, so nobody will cheat you, but I have no way of proving it to anybody. What I can say hallucinations do not bring wisdom and insight on that level. Hallucinations make you weird and more stupid in general; you just bubble words nobody understands and act weird.
It can not be an altered state of the brain, if you put MRI on me when in arupa jhanas it should show nothing, while in experience, you will feel extreme bliss that is beyond anything you can physically experience, which would prove it’s non-physical. I generally don’t like to talk about it simply because there aren’t people who can relate to it.