Question: How do I check if a component is not present, i.e. that a specific component has not been rendered?

Answer: 
Providing a slightly updated answer based on [the documentation for enzyme-matchers's `toExist`](https://github.com/FormidableLabs/enzyme-matchers/blob/master/packages/jest-enzyme/README.md#toexist). This will require you to install the `enzyme-matchers` package.

```
function Fixture() {
  return (
    <div>
      <span className="foo" />
      <span className="bar baz" />
    </div>
  );
}

const wrapper = mount(<Fixture />); // mount/render/shallow when applicable

expect(wrapper.find('span')).toExist();
expect(wrapper.find('ul')).not.toExist();

```
We use Jest and Enzyme, and I've found the only good test is to import the sub-component and test this way:

```
expect(component.find(SubComponent).length).toEqual(0); // or (1) for exists, obvs

```

I tried all the other answers and none worked reliably.