How do universities verify qualifications?
For entry level positions (postdocs, visiting assistant professors, assistant professors), we usually start with letters of recommendations and copies of publications, including the thesis. 

Usually one of the letter writers is the chair of the dissertation committee. That gives confidence that the dissertation is being completed or almost finished.

In one of my early jobs, I did have to send scans of my diplomas, but that was done after the job offer.

There's remarkably little authentication in academia. But that is likely because there is the perception that there is little outright credential fraud and because the consequences are so large.