Excellent question.

Unless the user reveals the info either intentionally or not, or if the forum admin chooses to look for duplicate IP addresses, there is no way to positively ID an alt account.

Personally, I consider that a failure of this forum. 

Sure, admin can't catch them all.  However, prohibiting alt accounts with appropriate negative repercussions would go a long way toward reducing their presence. It would also remove the gray area of having a "used only for good" alt account.

In a previous life, I spent a few years volunteering as a moderator on a fairly active forum.  I never saw an alt account being used for "good".  At best they were used to incite activity and typically they were at the edge (or beyond) of acceptable behavior. 

If I were king, I would ban alt accounts and brand the main account with "multiple personality disorder" tag for the next 12 months.