I have completed OAuth for a Salesforce customer, and I have their access_token and refresh_token.
But how do I know what API URL to call on their behalf? I.e. https://na16.salesforce.com/services/data/v36.0/ or https://na30.salesforce.com/services/data/v36.0/
I don't want to run any Apex code, or have them manually look at their address bar. Is there some way to get this through the API or OAuth process?