You got an invitation to visit one of those schools that you applied to. Before I go any further, I am assuming that you only applied to those schools that you are serious about attending! Well, if you can visit them all, then do so! You are about to make a decision that will directly impact your professional, as well as personal life for the next 5-7 years, thus, any effort you put into your decision is worth it. During your visit, make sure to talk to current graduate students to get the full picture, ask what equipment and resources are available to graduate students, check out potential housing options, as well as the community around campus. Finally, like Viridiana said “enjoy the visit”.
However, if you are like me and cannot take any time off to visit prospective schools. Then, I recommend that you email current grad students in order to get some feedback. Also, something that worked great for me was to track down someone (e.g. a friend of a friend) in my current institution that did their undergraduate studies at the institution I was invite to visit and get their input.