It sort of works like this:
(1) Look at their website, learn as much as possible
(2) Phone up and ask to come and visit.
(3) Visit, get shown around, ask lots of questions.
The phone bit is technically not essential, but gives a much better chance of somebody being free to show you around.
Unlike integrated, modular schools don't have set course start and end points, so tend to be quite happy showing you around, and having you start, whenever it's convenient to everybody - which basically means when it's convenient to you. And also means that scheduled open days aren't the obvious way to do it.
G