a fox smells his own hole

A fox smells his own hole (first) OR A fox smells its own lair first OR A fox smells its own stink first. (оne knows where they belong, and knows when they make a mistake) — ≈ знает кошка, чье мясо съела

Used to mean that if a person is accusing you of say, cheating, it's most likely because they've already done it or are thinking about it.

cf.
[рыльце в пушку]
[на воре шапка горит]