How to talk to previous lab members when applying to a postdoc? I hear that it's common to talk with previous lab members before accepting a postdoc position.
I was wandering what is the best way to approach these conversations.
Should I let the professor know that I am going to email their previous lab members? I don't think they would oppose, and if they do that would be very suspicious. But I'm not sure if it's common courtesy to let them know, or if it's unnecessary.
Also, when I email previous lab members, is there anything I should keep in mind?
Thank you.
Should I let the professor know that I am going to email their previous lab members?

There is no reason to do that.
I suggest it may be more useful to contact people who have left the lab, as they are less likely to feel a conflict of interest when describing their experiences.
Ask about the supervisor's management style.  Ask about how working in the lab advanced peoples' careers.  If you are unfamiliar with the location, ask about difficulties people encountered living there.