A good surrogacy agency has a highly detailed screening process for both surrogates and intended parents. The goal is to find a good match for both parties. So the real question here is how you find a reputable agency. And the answer is intensive research.
Agencies not only provide you with the best surrogate candidates, but they also control all of the money you pay the surrogate and will serve as a mediator if any problems arise. The best way to find a good agency is to ask someone you know who has already been through the surrogacy process. If this information isn't available to you, narrow it down to agencies that seem to be a good match and ask for references. Then be sure to check them all.