A lab visit is a performance. Everyone is on their best behavior, the schedule is curated, and you are talking to whoever the supervisor wants you to talk to. That does not make it useless. It means you have to know what to look for, because the warning signs are usually small, easy to explain away, and easy to miss when you want the position to work out.
None of these signs are proof of a bad lab on their own. But when several of them show up together, they are worth taking seriously.
What To Watch For
The supervisor cannot name a single former student's current job
Ask casually where past lab members ended up. A supervisor who has trained people for years should be able to rattle off a few names and positions without effort. Vague answers, long pauses, or a sudden change of subject usually mean something.
Current students seem reluctant to speak without the supervisor present
Watch the body language during the visit, not just the words. If a current student hesitates before answering, glances toward the supervisor, or gives unusually polished and positive answers, that discomfort is information.
The supervisor talks mostly about themselves, not your project
A good supervision conversation should be at least as much about your interests and career goals as about their lab's output. If every answer circles back to the supervisor's own achievements, that is a preview of how attention will be distributed later.
Working hours are described as flexible, but everyone is in the lab at night and on weekends
Pay attention to what people are actually doing, not what they are told to say. If the stated policy and the visible reality do not match, the visible reality is what you should plan around.
Authorship and credit are discussed evasively
Ask directly how authorship is decided on papers. A supervisor with a clear, consistent answer has likely thought it through fairly. Vague or shifting answers here often predict disputes later, when you have far less leverage than you do today.
Trust the Pattern, Not the Moment
What matters is whether several of these signs show up together, and whether they match what you hear from people who have actually worked there. A lab visit gives you a few hours of curated impressions. Former students who have nothing left to gain or lose by being honest with you can give you the rest.
If something about a lab visit felt off and you want to know whether others noticed the same thing, the most useful thing you can do is ask former students directly, not just the ones the supervisor introduces you to.
If you have worked with a supervisor and want to share what you found, honestly and anonymously at rateyourpi.com. That information is the most valuable thing you can contribute to the next researcher making this decision.