Look for gaps on work experience. The person you're interviewing may have two or more years experience, but maybe that happened four years ago. So now they're a little rusty, in addition to getting used to your particular shop's operation and procedures. Don't hire anyone on the day of the interview. Call them back for a second interview if necessary to further aid in your decision.
Some applicants lie or puff up their job description.
I can't say enough about liars and thieves. These people will milk you for a lot of money. It can be thirty days or any time period. Most are ninety days. You can get a good idea within three months if a new employee will work out or not.