You should look at where your prospective superviser's students and postdocs end up after leaving them. Talk to these people if you know any of them. Also, look at the group publication list. Is the group productive? Do they have strong collaborators?
I have been told to use the postdoc to learn some new technique, or whatever. Your goal should be to make the most of what you already know. It's more important to improve your academic record than your knowlegde at this point. You will not be hired as a faculty based on what you know, but based on what you have published and where.
Having that in mind, try to see, given your current skill set, how good fit you are for that group and department. If you can collaborate with enough people, you'll get more publications. You need to have access in that department to people with similar background to yours. If that's not the case, you will be isolated, and your productivity will drop.
Some red flags I'm thinking about:
- postdocs and students leaving before the end of their contracts,
- postdocs and students ending up in poor positions after leaving,
- not enough people in the group/department with similar background to yours,
- poor publication record of the professor and postdocs compared to similar groups
- lack of strong collaborators.