How would you know they had one? What is the process for disclosing or discovering non-competes? Ostensibly part of a private citizen's employment contract, that's a confidential document.

So what basis would a prospective employer have to request a copy, and how would said third party verify its authenticity?

You ask them. And you probably have little interest in seeing a copy in my example. Just if there is one.

Yes, they can presumably lie to you. But if you then so much as get a letter from a lawyer at that point, you fire them at a minimum.

Most contracts I’ve signed have language to the effect of “the employee is not bound to any covenants that could restrict his ability to provide his services to the company”, which would include noncompetes.

Of course, noncompetes being unenforceable make for a good argument that one is not, in fact, bound by that covenant.

In practice, I’ve always just answered “no” to the question on this basis. If any former employer behaves in such a way as to get me fired from my present job, I’m rather optimistic about my prospects for legal remedy.