Your mileage may vary, but here is what I do.
I try to pick about 15% of all new loans for review (but your institution's size may make that impractical). When I review loans I look for negative traits in pulling the sample (ie low credit scores, collateral types that are suspect -1979 Pinto securing a $10,000 loan-, lenders who specialize in one type of lending branching into another type, etc.).
It took a while for it to sink in with me, but it is important to have a good rapport with the lending staff when you do reviews. I try to stress that we're all in it together and we have the same goals: clean portfolio, no unnecessary exceptions, good compliance, etc. I've also learned that the lenders can usually provide great information about customers when you talk to them, but they can't always convey that information when they do a write-up. Be fair, be positive, and use neutral language if you do a summary as part of your review.
I would also try to get a feel for the concerns of the board and internal management about portfolio weaknesses and obtain a heavy sample of loans from those areas.
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