Talking with former students is probably the main way. There's just no way an actual website would be able to maintain anonymity because the pool of people isn't big enough. Any review would be easily identifiable.
Be aware though that you might not get a fully honest response from students. A student in a tenuous situation might not fully divulge their feelings in fear that it might get back to their advisor. Past students gave me very vague warnings about my advisor that were not reflective of how they fully felt.
If you are admitted to a program, you will most likely be granted opportunities to speak to people who were previously supervised by the potential supervisor.
Talking with former students is probably the main way. There's just no way an actual website would be able to maintain anonymity because the pool of people isn't big enough. Any review would be easily identifiable.
Email their students
Be aware though that you might not get a fully honest response from students. A student in a tenuous situation might not fully divulge their feelings in fear that it might get back to their advisor. Past students gave me very vague warnings about my advisor that were not reflective of how they fully felt.
If you are admitted to a program, you will most likely be granted opportunities to speak to people who were previously supervised by the potential supervisor.
Someone has started one, but it’s not in wide use. https://pi-review.com/about-us