in reply to Is it multi-user?
Hmmm... I'm not sure there's any general way to determine that, without ennumerating and testing for specific systems. Why do you need to know? Do the tests have an adverse effect if they're unnecessary? Are they run often enough to impact performance? What happens on a single-user system (e.g., BeOS, MacOS 9, or DOS), that you want to avoid?
Also, how are you defining "single-user system"? Does Windows 98 count, even though it has a concept of user accounts? Why? Because only one can be active at a time, or because the filesystem doesn't have a concept of file ownership? Which? If the latter, would a Linux system count if the files in question are on a vfat or UMSDOS filesystem?