> explicitly directly ask to see offer letters.

and you tell them that the company do not want their offer to be revealed due to confidentiality reasons. The first employer would need to take your word for it. And if they deem you lying, they can always stand hard on their offer, and risk losing the candidate.

Real or not, I’m not sharing an offer letter with another prospective employer.

I have shared them with my current employer to let them know the market and what I was considering.

Oh well? If the employer is negotiation hard enough that one feels the need to make up another offer, a deal wasn't going to be reached anyway.