They can't really know for sure who is going to hyperstim and who won't. The predictor they go by is high E2 and multiple follicles. My donor did not hyperstim but they thought she would due to her high E2 at trigger. Clinic did try to cancel the retrieval, donor insisted based on her past pattern of high E2 without problems.
I love our donor very much and did not want her to be in any pain, it was a scary time.
You asked for what questions to ask your clinic, I would honestly try to speak directly to an RE about it, not the nurses. Ask if there is an E2 level cutoff or guideline where she would possibly be cancelled, and what can be done to avoid it. Ask about the effects of coasting stims.
I was NOT happy with how my clinic handled the whole thing, it was chaotic. There was a successful pg from that retrieval, but I don't think my clinic handled it well.