To answer my own question
There are some functions that are in React Util that you can use to search for elements so in my case it was like this
let component = TestUtils.renderIntoDocument(
<Heading />
);
let h1 = TestUtils.scryRenderedDOMComponentsWithTag(
component, 'h1'
);
expect(h1.length).toEqual(0);
since no arguments are passed to the Heading then no h1 elements should be rendered on the DOM