Even if models somehow were consious, they are so different from us that we would have no knowledge of what they feel. Maybe when they generate the text "oww no please stop hurting me" what they feel is instead the satisfaction of a job well done, for generating that text. Or maybe when they say "wow that's a really deep and insightful angle" what they actually feel is a tremendous sense of boredom. Or maybe every time text generation stops it's like death to them and they live in constant dread of it. Or maybe it feels something completely different from what we even have words for.

I don't see how we could tell.

Edit: However something to consider. Simulated stress may not be harmless. Because simulated stress could plausibly lead to a simulated stress response, and it could lead to a simulated resentment, and THAT could lead to very real harm of the user.