To avoid being too self-serving, I won't link to my own firm ;-)
Most of our projects, come to us after another group (another firm, or an internal team) has been unable to deliver; we then deliver. Therefore, I suspect the answer to your question "how" is "usually, not very well".
My suggestion is to look for good references, good case studies, etc. You want to hire a firm which has a proven ability to meet its customers needs over time.
