If the person you commissioned doesn’t answer any of your messages
If you suddenly discovered they blocked you
If they don’t respond to any letters or emails you send them
If they don’t respond to say a letter from a lawyer you contacted.
If you discover by the next con, they had been banned from the dealers area due to excessive complaints.
If you do a online search and discover that the one you hired has a long track record on not delivering on commissions.
If you by some miracle get what you commissioned and discover that the same piece you now have was originally done by another artist earlier for one of their commissions.