Notary signing agents are instructed to always be polite and courteous, and to never address the borrowers by their first name, even though they may insist upon it. So throughout the signing it is always 'Mr. Borrower' or 'Mrs. Borrower'.
But it's not always 'Mr.' or 'Mrs.'
There have been times when the borrower is a doctor. And I have done closings for pastors of a church. In situations like this I want to address the borrower by their appropriate title -- 'Dr. Borrower', 'Reverend Borrower', or whatever is appropriate.
Usually the location of the closing will give a clue. I once did a closing for a dentist at his office. I addressed him as 'Dr. ...' But the location is not always an indicator. I did a closing at a church recently. The borrower worked there, but he was not the pastor.
Sometimes the 1003 Loan Application can be used to let the signing agent know what the borrower's occupation is. But the occupation won't tell you what degrees the borrower has. If he's an engineer, he could be a Ph.D., in which case he could be called 'Dr.', even though he is not an M.D.
Borrowers might be very sensitive about how they are addressed. They may even be insulted if you address them as Mr. or Mrs., if they prefer to be called 'Dr.' or 'Reverend'. So the best thing to do, when in doubt, is to of course ... ask.
P.S. Of course, there are certain people whom you always address as 'Mr.'