Rails tests with various integrations

I have a rails app that interacts with an external api (Salesforce) that relies upon external data sitting in a remote database. I've written a wrapper that wraps this code so that users can just call get_by_id(id) instead of writing the corresponding sql query.

I want to test this code, and I am not sure how I should go about it. Should I be hitting the Salesforce backend database for the tests, calling the real methods? Or should I just mock the results of the method calls? I am perpetually confused by what I should test...