That’s great that you want to research departments, it’s a great way to determine if a school is a good fit, and a very important consideration (and one that in general prospective students/parents probably don’t do as much as they should).
I think first you should have some idea what level of school you can get into (and afford) – no sense researching top schools if you have basically no chance to attend. Your GPA and test scores (and financial situation) can give an idea of that.
Next, I suspect there’s a lot more information on department websites than you suggest. Just find a school’s website for the department of interest, and start poking around. One of the the key things you should look for is a faculty list – that’ll give you an idea of the size of the department. Often they’ll show things like what degrees the faculty have and from where, their publications and research interests, and other things they’re involved in. You might want to pay attention to the dates listed, to get an idea what people have been doing recently.
Also, if a department is some kind of combined department (e.g., at Williams it’s Art History and Studio Art), beware that it might list people not directly in your area of interest. And I’ve noticed that often these faculty lists are actually “faculty & staff” lists, so they include administrative and other positions, as well as Visiting and Emeritus positions (I suspect they do this to make them appear larger).
Besides these faculty pages, there might be other pages showing research and activities that the department, faculty, and students are involved in. You already mentioned courses – that can be helpful too, but pay attention to how frequently and how recently courses have been taught.
This will take a little bit of work, but it will be worth it.
This is a good general strategy for researching any kind of department. Another possibility is if you know someone in the field, you can get information and recommendations from them. And there are probably other sources of specific relevance to art history (or other departments).
Another question: How certain are you that you will major in art history? If there’s some chance you will switch (and many students do switch majors), you should not base your decision entirely on the strength of the one department.