Irene: If they keep beer in the cold room. In all seriousness, I think the most important thing is to agree on expectations.
Yuhao: You can’t answer this question before meeting the people in the lab. That’s why I always request a meeting when I first make contact with a professor, instead of directly asking for a position. Ask the professor to show you around the lab and introduce you to the lab members. You usually get a pretty good idea after your first visit.
Elaine: I usually go into a lab not knowing whether it’ll be a good fit. What I’ve found makes the biggest difference, once you’ve finally settled, is your rapport with the rest of the lab members. I’ve been in labs where the other lab members are friendly, accommodating, and very helpful – these tend to be the labs where you have the best time. I’ve also been in labs (like my current one), where the other lab members tend to keep to themselves, and do their own thing. It’s not as fun, especially because I’ve had other lab experiences with which to compare it, but at least for me, wasn’t something I would have been able to figure out before actually coming here.
Anonymous cynical person: In my opinion, I don’t think you will know whether a lab will be a good fit or not. Any undergraduate position is hard-earned, so being picky when you are just starting out is the wrong mentality. I think you’ll find out once you get to the lab – whereupon you’ll have an easier time getting another lab position (having put your foot in the door). Moving labs is definitely an option, so I wouldn’t worry about whether the lab will be a good fit when choosing your first lab.