The method I described using stat will work on every *nix system, which set incidentally happens to be the set of systems on which mountpoints are a routine issue.

While Windows NT does (or did) have a similar feature, even including the ability to mount another volume atop an NTFS directory, it is very rarely used on that system. In the modern era, the existence of mountpoints strongly implies POSIX or possibly Plan9, but Plan9 is relatively rare and it is generally safe to assume that anyone asking questions pertinent to Plan9 would mention its use. I only mention the "modern era" qualifier to exclude pre-POSIX Unix, from which POSIX inherited its filesystem model.