In the stat there's a hint...

Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
      print "$file is executable NFS file\n";
    }

(This works on machines only for which the device number is negative u
+nder NFS.)
[download]

...since Mac uses a BSD variant as OS and NFS is a Sun contrivance in its BSD time...

I don't own a Mac, and automount is out of scope for me for some years now. But anyways, parsing /etc/mtab or /etc/fstab or such, getting the local device numbers via stat of the mount points should give you a clue: anything not in that set of numbers is from remote, and automounted.