My current farrier's suggestion is to ask potential new farriers, "If you couldn't do my horse, who would you recommend?" The idea being that there are probably one or two people that the farrier community in the area generally recognize as as being very talented and probably good bets.
Other things I will be looking for myself is continuing education- do they go to clinics, competitions or classes to improve their skills? Finally, I prefer someone that has the ability to hot shoe- not necessarily because every horse needs shoes or that the ones that do need custom shoes, but because IMO shoes, along with good trimming, nutrition, and boots, are just one more tool in the box that can be pulled out when appropriate to keep a horse sound and comfortable.
So, what are your favorite tips for finding the right person?