My main red flag would be someone who doesn't understand your business and doesn't ask the salient questions as they relate to PPC.

I would come armed with all of the business metrics; who your main competitors are, what CPA you will want to achieve from PPC, how you will measure success, what tools are relevant to understanding your particular market etc. Ask the right questions then see how your candidate responds.

Let me know if you would like a call to discuss what these metrics and how best to plan a strategy around PPC.