Watch out for people who are fresh out of one of those certification courses and think they know everything. Also, watch out for people who have never taken a class, but still think they know everything.
Like any good Kung-Fu Master in a kung-fu movie said, "The first step to enlightenment is to admit that you know nothing".
To me (as has been echoed numerous times in this thread) the most dangerous person is the person who knows less than he thinks he does.
Because this is a pretty basic position, I would hire a person who knows less, but knows exactly where the limitations of their knowledge-are (and is honest about it). At least I can work with that person and increase their knowledge.
Pull some examples from your tech support database. The weirder the better. Give her/him exactly the same information that your existing technicians had access to. Purposely leave out some crucial information, maybe the way your network is configured (such as static IPs or DHCP)
Ask her/him what he would do: not necessarily providing exact solution, but more the mental problem-solving process. Remember, in a Help-Desk position, you're after problem-solving skills, and not just technical skills.
If they ask some intelligent questions, like "What kind of method is used for IP Address assignment?", then they've scored some crucial points.
When s/he's done, tell them how the problem was really resolved (if different), and ask them to describe which method was better.
The answers that you get will vary, and the "mine is better" or "yours is better" answer is not what you should pay-attention-to. This question will always fluster the interviewee, and that is the whole point: In their mind, they're worried about disappointing you, failing the interview, or seeming too cocky if they say "Mine is better than yours". The point is to evaluate how they can articulate themselves and think clearly under pressure and uncertainty, because these are the same kinds of pressures they will encounter if they cannot quickly resolve a situation in the field, or don't understand what's going on in a particular issue.
Being articulate is very important, and making sure that the technician can accurately describe things to you (since you're no-longer on the front-lines) becomes crucial.