Don't ask the barber if you need a haircut

Don't ask the barber if you need a haircut — a simple reminder that asking someone with a vested interest in the outcome isn't likely to give you an impartial answer.

Except that while we might reasonably make decisions about our hair with few consequences to anyone but ourselves, when we work with professionals they may well see or hear what we don't. It might be that asking a professional to review what you're doing is exactly what you need to tell you what you didn't and couldn't think to ask.

The phrase has been famously shared by Warren Buffet though it appears in several similar earlier forms.