That's a real common mistake.
References are the best way to do that.
Whenever I look for a consultant, I always ask for references, and I want real candid, honest, complete references. I want to know if they were able to service the client well and if the resources were there to service them expeditiously and with some talent. The best predictor of future performance is past performance. The key is to work very closely with your consultant. You should have a very clear picture of what you want that consultant to do.