You should use the same criteria you use in selecting any other important professional.
You could certainly ask an individual. Word of mouth is often the best source of referrals, but there is no shortage of information out there about qualified personal injury attorneys. Certainly, you would want to look for the same qualifications and use the same criteria that you would utilize in making any other hiring decision. You would want to know what type of work someone does, how long they've been doing it, and just generally, what their body of work looks like.