Well, the most basic answer is that you could look for certifications. However, that only takes you so far. Any PPC professional worth his/her salt also has experience with PPC management software (Kenshoo, Marin, Acquisio), etc. Ask what experience they have and what they like dislike. I have my favourites :) More over any good PPC expert has their own strategies for managing and structuring campaign. Ask what strategies they use. GIve sample cases and how they think. However, you really have to know what you are doing to hire well. I would suggest you get a PPC expert to do the interviewing and screening for you. Identify 2/3 good people and you hire one that you like for the personality fit.

Good Luck :)