Using sinon to test if a function is being called on LitElement object

Question

I have a class that extends from LitElement. There is a function test() in the class that calls a function outside the class isClientsEqual. I want to test that the function isClientsEqual is called when test is being called. How can I do that using sinon?

Here's my class

function isClientsEqual() {
    console.dir("test");
}

class HK extends LitElement {
     private test() {
         let isEqual = isClientsEqual();
     }
}

Notice how isClientsEqual is outside of the HK class.