What should a student look for in an undergraduate thesis topic/supervisor?
I am in math, and have some experience with undergrad research (from both sides), and there are lots of different ways you could have a good experience with undergrad research.  What would be good for you will depend a lot on your individual situation (mathematical maturity, background, independence, etc) and your goals (do you want to get a paper out of it? just get some research experience? learn something new? help your case for grad schools?).


For getting into a good grad school, what's most important is how well prepared you are in terms of background and how much potential you show.  This is evidenced by your transcript and recommendation letters.  Doing novel research or writing a paper is much less important (unless it's much more impressive than typical undergrad math research).  (Note: many undergrad theses in math are just an exposition of an important topic.  Some of these are very good.)  So in terms of grad school, the most important thing out of this project will probably be (1) what you learn, and (2) the impression you make on your supervisor, who should write you a recommendation.
For being able to get a paper out of it, research is never certain, but it's important that the project is well thought out beforehand.  More open-ended projects may sound interesting, and teach you a lot, but may not be conducive to having a finished project in the time available.  I generally find ones that involve some calcutions/computation have a higher rate of "success," in the sense you should at least be able to do the calculations, whereas projects that require you to develop some theory may just leave you completely stumped.  (Though some types of theoretical projects where you can follow the method of another paper closely also have high rates of success.  Conversely, try to stay away from projects of the form "look for pattern in this data.")  Hopefully you can get a sense of how well thought out the project is from the description; also knowing how successful the supervisor was with previous students may suggest this person gives high-percentage or low-percentage success problems (but if you're more concerned about grad school, where the students went for grad school is more important than if they published papers).
For learning things (which helps with grad school), a project where you have to learn some theory (even if just to do computations) is better.
Both to make progress and to learn things, an accessible advisor (say is willing to meet you once a week or so) is better.


Apart from that, I would say put some value in your general impression from talking to the potential supervisor.  If you have a chance to talk to former students of the supervisor, that could also be useful.