Talk with students in group: Are they happy? Do they like what they do? What is the environment? How do they describe their advisor and how he/she interacts with graduate students?
You’re going to be spending quite a bit of time with your labmates so make sure these are people you can work with.
Don’t wait too long to join a group (second year is a good time)—if it doesn’t work out, you can switch.
Learn how various projects within the lab fit together. If you join the lab will you work on a single big project or several small ones? And are those projects risky or likely to be successful? Is there flexibility in terms of changing projects? Are certain projects a “favorite” of the advisor? (This can mean you get a lot of attention, it can also mean you’ll be under more pressure and have less say about its direction.)
Take a long term view of the research—how do the skills you will develop fit with your career goals (e.g. instrumentation, programming, fabrication experience)?
Recognize where your advisor is in his/her career. Tenured professors tend to be less hands-on then untenured professors. But the latter has less experience (if any) advising graduate students than the former. There are several older professors in the department that are close to retirement or already retired; this brings it’s own set of benefits and drawbacks.