A better question may be "how do you know someone is a good lender?". By this I mean too many mortgage brokers will gather a minimal amount of financial information and issue you a pre-qualification letter. Many times these letters are not worth the paper they're written on.
In my opinion as a real estate broker, if you feel they're asking too many questions about your moving plans, asking for too many financial documents they're a good lender.
Making sure you qualify for the home you're looking for protects you from being disappointed if you fall in love with a home only find out there's something in your credit report that needs too be addressed after your offer has been accepted.
A good lender will make sure from the beginning that you'll be able to afford and close on the home you want, and be able to address any credit or finance concerns early.
I only use lenders who are proactive from the beginning. If you'd like to contact any of them please call or email and I'll forward you their information.
Good luck to you.
Forbes Realty of South Florida
West Palm Beach FL