One of the open source software I'm contributing to has recently started using features that are only available on OS X 10.7 and later. We'd like to detect at startup time (or preferably before the program is even started, if some DMG metadata allows for that) if a user is using an older version of OS X and display a message telling him to upgrade his OS (or use an older version of our software).
What is the best way to do this? I'm not an OS X developer so please keep it as simple as possible :)