This website shows how a consent-receipt would work: when you sign up for this imaginary service, you agree to its terms of service, that is, you give your consent. Once you do this, a download link will appear where you can download a receipt of the consent you just gave. This is a machine-readable JSON-LD document, for you to keep, specifying what you agreed to and how to revoke the consent when you no longer do.

It also contains a timestamp and a cryptographic signature (the signature is a fake one in this demo). The format is described on http://opennotice.org/spec/receipt-0.1.