...background check.
Yep.
The best defense is to never forget two things: 1. Loose lips sink ships, or my ass. 2. The only sure way two people can keep a secret is that if one of them is dead. And as a bonus: Even a fish would not get caught if he kept his mouth shut. That one, however is not foolproof, there are nets and seines. But it does cut down the odds considerably.