How to get a field name from an instance variable

Please tell me about how to get a field name from an instance variable.

public class MainClass
    {
        public void output()
        {
            Action valAction = new TestClass().testAction;

            //I want to output string 'testAction' from valAction variable here
            Debug.WriteLine(valAction.????);
        }
    }

public class TestClass
    {
        public Action testAction = () => { };
    }