Everything I could possibly say is summed up very nicely at the bottom of this page:
http://pages.physics.cornell.edu/~larri ... chool.htmlScroll to the bottom and there's a section on "Visiting Graduate Schools." I asked most of the questions on that list, and I found it to be extremely helpful. I visited a school, and I just wore what I would normally wear (for me, jeans a t-shirt). No one seemed to notice or care as most of the current students and faculty were wearing the same thing.
About your last question regarding forcing people out with a masters just so they have the TAs: I would ask about qualifiers. I asked both students and faculty about the difficulty, how many people passed them, etc. That is somewhat of a more subtle route (for the school I visited, I wasn't really concerned about it that much). If you're really concerned about it at a specific program, you could take a more candid route with the students and ask directly if that happens.