const wrapper = shallow(<MyComponent />);
expect(wrapper.find('.foo')).to.have.lengthOf(1);
expect(wrapper.find('.bar')).to.have.lengthOf(3);
// compound selector
expect(wrapper.find('div.some-class')).to.have.lengthOf(3);
// CSS id selector
expect(wrapper.find('#foo')).to.have.lengthOf(1);