There are a couple ways of detecting with vimscript if you are on a mac. This can be useful if you are writing a plugin with OS-specific functionality. Here are two ways to make that check.
if has('macunix') || has('mac') || has('osx')
...
endif
Alternatively, you can use Vim's system() function to execute unix's
uname command. This command will give you the name of the operating
system. In the event you are using a Mac, the result of uname should be
Darwin. The following regex match is a good way to make this check.
if system('uname') =~ "Darwin"
...
endif
See :h has(), :h system(), and man uname for more details.