Talk with the graduate students who are now in the programs you are considering. Are they happy?Hmm.... yes, its definitely a good idea to talk to graduate students before choosing a program, but "happy" might be a bit too much to expect. (And if you do meet graduate students who are "happy," do them a favor and don't report them to the director of graduate studies).
My advice on grad school is here.
Update: Chris Blattman adds to Mankiw's list.