I think perhaps an even nicer way to do that would be to make the scuba gear and open the mariachis and try to activate them anyway. Then at that point, see if you are missing an instrument for one of them (or more than one), and tell you. After all, a new stringed item could come into the game that works, and no one realizes it for a while, and it wouldn't get added to mafia's list (i haven't looked at how it decides if you have a guitar or not).
That said, i've never had it do that to me yet when I had a valid guitar type item.