A found wallet with cash in it is a lie detector that fits in your pocket.

Certain situations tell you something reasoning cannot, because there is no consequence to argue with and nobody to perform for. Nobody would ever know, which strips away every reason to behave except the one that actually matters. Whatever you do is the honest answer. That is unsettling and also clarifying.

Hand it in intact, including the part that would be easiest to keep. Do it quickly, before the negotiating starts. What you get is a specific and lasting piece of information about who you are.