How do I know that the invoice was marked 'paid' within SkyBox?

Once Reveal marks an invoice as "Complete", you'll see tag in the invoice with "revealmarkets-paid".  

You can run an invoice payments report to see all the invoices Reveal has marked paid.  Just search by the username you gave Reveal when the API key was created.

Skybox example:

Look for the tag in the Invoice Tags field.

You'll also see the "Payments - Mark as" field updated.