I also strongly recommend that you ask your Mortgage Professional if they will guarantee their Good Faith Estimate closing costs...if they won't...there's your first clue!
That being said, references that a loan officer gives may be cherry-picked (usually are) and may be "vanilla" transactions that did not require any underhanded treatment. Likewise, I've heard of lenders who collaborate with unethical real estate agents, so that one is sketchy, too, unless the agent has a strong reputation.
Here's a novel idea - also ask for a contact at a couple of the lending institutions to whom that loan originator sends loans. They ought to know if they submit fishy loan apps.
Unfortunately, there are no good, easy answers. Due diligence on the part of the borrower is still the best approach.
I'm sure Mr. California Mortgage Broker has the same horror stories to tell - someone went to a cousin, friend, friend of a friend, someone that they trusted to watch out for their best interests, only to be taken for a ride.