How should a family office underwrite a first-time sponsor?

Underwrite the operator, not the deck. Concretely: pull title on every deal they claim, call the lender on their last two deals (sponsors sign off on this if they're real), and ask for the capital call and distribution history — dates and amounts, not IRRs.

Then the soft test that predicts everything: give them a small problem on a Friday and watch what happens. Speed and honesty of communication under mild stress is the single best predictor of behavior when a deal goes sideways.