How to test if a functions is called in React?

Working template of the project: https://codesandbox.io/s/blue-currying-3me1t

The only test I have set up is in src/App.test.js to check that a name is rendered. I am trying to setup another test that checks to see if a function is called before the buttons are generated.

In FontAwesome.js there is a registerIcons() function that registers all of the icons needed for the website. I call this function in Buttons.js as it is the place where I use the icons.

I want to create a test in App.test.js that checks to see if registerIcons() is called before the buttons are created in Buttons.js.