Interviews are complex, but I view part of the process as negotiation. One of the principles of being an effective negotiator is knowing what you desire to be the future relationship between the parties. Typically in the technology space, I don't like burning any bridges.

Thus, I start the interview process out by saying that I value and appreciate directness and honesty in all of my dealings. In keeping with that, if at any time I feel I have enough information to know that this will not be a good fit, that I will be honest about that and we can part on good terms and not waste each other's time. I also ask in return that they be up-front and honest with me (don't bring me in for interview after interview just as a charade when they know they're giving the job to Fred anyway).

I've used this tactic a handful of times and it has never failed to get a positive response. On a negotiation level, it may also give me a slight edge because I broached the topic first and raised the possibility of not seeing the process through to the end, something many interviewers forget is a possibility.