From a teaching perspective, lefties are a minor pain. I certainly suggest that newcomers to the instrument at least attempt to play as a conventional righthander since that provides much more material and gear, fewer restrictions later. If for whatever reason the student (or their family) insist on lefthandedness, just do what you usually do but with the mirror image points others have mentioned.

For the record, I've never found the "test" mentioned above by others to indicate much since the uninformed, uneducated and inexperienced potential student of either hand will show inconsistent results.

Peace, Mooh.