I'm not sure if this is a good answer or not, but if you're interested in a particular subject that an advisor has worked on, and I mean "particular" as in a very narrow aspect of the professor's research, then maybe you could ask him/her if he/she still does work in this area, or something similar. Or you could even just express your interest in that particular aspect of their research.
For example, I have mentioned to a potential advisor (very nerve-racking, even via email) that I am interested in a particular function that he has done research on, and has even published a big paper on. There's a couple of questions I have about this specific function that I could ask him, but decided not to due to my lack of background knowledge.
However, I am also interested in hearing such answers! Also, do many colleges in the US do interviews before admissions? I keep hearing about these interviews from professors from other applicants.