Learn to read tattoos and fire the ones with gang signs. That will work until they decide not to mark themselves. The method could last for a while as the gangsters take a lot of pride in their tattoos.
The MIL already has rules about tats.
It may be more true in Britain but the psychiatrist and widely acclaimed author Theodore Dalrymple said that although not all people who have tattoos are criminals, ALL criminals have tattoos.
Gee.....wouldn’t that be called racial profiling?
Just kidding!