I think you overestimate the difficulty of determining such a thing. In fact it is generally agreed by psychologists that thought does not require language. Some arguments include: pre-linguistic infants’ demonstrating reasoning and surprise when events defy expectations; people with aphasia continuing to display advanced cognitive abilities; mental calculation which can be done without the use of internal words; tip-of-the-tongue phenomenon, when we know what we are thinking but can’t find the word; spatial reasoning — and so on. These are compelling arguments against the idea that language is a precondition for thought. Indeed, how did we get language started at all if we had no pre-linguistic thoughts to label? My argument is that the idea of “reality” may be premised on primitive experiences of dreams and imaginings and their clear difference from the real world, and that it is plausible that a person without language could indeed comprehend that a simulated world was not real through some non-linguistic demonstration, though I do agree with Paul that if the real and simulated worlds were experientially indistinguishable, then any person, linguistic or not, would have trouble concluding that the simulation was not in some sense as real as real.