I'm working on an Folder Action to automatically create some symlinks, and needed to account for a finite number of temporary files, which requires that I know if a symlink is valid or not.
Apple's Developer Tools ships with a utility called GetFileInfo that does just the trick.
/Developer/Tools/GetFileInfo -aa _SYMLINK_ >/dev/null 2>&1 ; echo $?
This will return 0 if the symlink (_SYMLINK_) points to a valid file, and as near as I can tell, 3 if the file does not exist.
Bonus
On Linux you can find the filenames of all broken symlinks in a given directory using find/xargs/grep/sed with
find . -type l -print0 | xargs -0 file | grep "broken symbolic" | sed -e 's/^\|: *broken symbolic.*$/"/g'
The only piece that doesn't work on OS X is the sed regexp (which I'm too lazy to fix), but you can kind of work around that with cut, as long as your symlink filenames don't contain a colon.
find . -type l -print0 | xargs -0 file | grep "broken symbolic" | cut -d':' -f1
