MacProcess, sure the Identification of Mac OS users it is the custom code modification because there is no such a functional in the default software.
I can assume that it is possible if to parse the User-Agent header. It contains various basic information that can help you identify visitors and their accessed browser, device and OS attributes. But you should be experienced in programming.
If you are not familiar with the code modification or plugin development, it would be better for you to find a developer who can help you. You can do it here:
http://www.oxwall.org/market/specialists