When someone susses something, they figure it out or understand it.
If someone susses something, it is usually good because it means they are smart and clever.
Examples:
She susses out the answer to the riddle.
Can you suss out what the secret message says?
He susses that the cat is hiding under the bed.
The detective susses out who took the cookies from the jar.
I suss out the trick and win the game.
The scientist susses the mystery of the missing socks.