Mocha, Chai & Sinon: Checking internal working of an API

Let's suppose I have a POST endpoint /user/:id and this endpoint, internally calls a function getUserData(id) and then returns the result to the caller, which in-turn returns the output, after JSON.stringifying.

Now, I need to ensure that getUserData(id) is called, for at-least once. How can I stub / spy getUserData(id) function, when I am using chai-http to make a post request to the server? Is it even a correct approach?

via Rajat Saxena