What are some good questions to ask current graduate students when visiting schools?
You'll want to get a sense of their feelings for:


The graduate program


Easy to work with regarding customizing the program to their specific needs (i.e., taking courses outside their specific area if necessary)?
How easy/difficult was the process for joining a lab?
Have they found the staff easy/hard to work with?

The advisor


Attitude towards students (respectful/distant/slavedriver)
Presence in the lab (micromanager/occasional presence/absentee)
How organized is the research?

The university as a research institution


Easy collaboration between departments (in their opinion)?
Availability of course offerings (from their experience)?

The city as a place to live