At the end of the interview you should be ready to make a sharp decision about the candidate - Hire or No Hire. Send word back to the recruiter and write two paragraphs of commentary to back up your decision.See Joel 1 for links.
Never say "Maybe, I can't tell." If you can't tell, that means No Hire. If you are on the fence, that means No Hire. Never say, "Well, Hire, I guess, but I'm a little bit concerned about...". That's a No Hire as well.
Remember this: it is much better to reject a good candidate than to accept a bad candidate. A bad candidate will cost a lot of money and effort and waste other people's time fixing all their bugs. If you have any doubts, No Hire.
Don't worry if you reject a lot of people. Never lower your standards no matter how hard it seems to find great candidates.
Joel On Interviewing 3: Be Decisive