Everyone has you covered on what to wear, so here are some tips on what to ask. Time with current grad students is huge – conversations with grad students helped me immensely in making my decision. It's a great opportunity to hear how a department really works. I'd ask about the quality of the course offerings, the department culture, how well the stipend stretches in the area, what non-stipend supports look like including travel funds, professional development funds, etc. I believe you are also an Americanist, correct? If you are talking to other Americanists ask how well the university supports American based archives trips. I've found that sometimes people can write off American trips as being cheap (and they are compared to international research!), but I still have to hole up somewhere for weeks/months at a time and that costs money. Feel free to DM me if you want to talk more!