what part of MERN stack app should be tested?

I got simple MERN stack app with form in react, which passes fields to redux state. Redux connects with node/express on backend which saved data in MongoDB.

Form need to contain valid email (which is checked in react).

My task is to test app, but I'm not sure what I can test and which tools I can use to accomplish this task.

My first idea is to test react component function that validates email and another one that check date format.