You notice on the way in that the code now on the whiteboard isn't yours.

I might casually ask how many people they're interviewing that day, and if it leads to talking about what's on the whiteboard, then I might give my opinion about what's on the whiteboard. Then again, what's on the whiteboard might have been written by your next boss (and not another interviewee), and you may have to be careful about what you say about the code (if you want the job, that is) :-)