Clothing: Pants, shirt, vest/tie/both unless told otherwise. That's what I've seen and done as both a law student and Finance PhD student, though those tend to be more conservative (clothing-wise) professions. Bring a blazer or a sportscoat in case the standard trends more towards business than business casual.
I had this concern when looking at law schools. I asked the professor. He said he had no plans to retire, but circumstances beyond his control altered those plans. You pays your money and you takes your chances, but he didn't seem offended. I'd suggest being open about it. If you meet with the professor and you have a really good rapport built, it's much easier to tell him (her) that their presence is a big reason for your decision. You then can ask if they are *currently actively considering* leaving the school. That will at least give them the chance to tell you if they're planning on taking the 30 year package in 2 years or flying off to teach at Arizona/Florida State. I'd view asking about the institution's tenure policy/rates if you're wanting to work with a junior scholar in a similar light. If you know a current grad student or faculty member, it might be more comfortable to ask them, but a professor's hardly going to get offended because you want him to influence your career and would like to know how likely it is he'll be doing that. .