How to test a React ref with a callback?

Enzyme docs contains how to test a node having ref with wrapper.ref('nameOfRef'), but this only works for refs having just a string value like, if I have a node in React:

<span ref="secondRef" amount={4}>Second</span>

Then its test would be written like:

expect(wrapper.ref('secondRef').prop('amount')).to.equal(4);

But if I have a ref with a callback, then how to test it? Enzyme docs [1] does not says anything about this. For example, if I have a node with a ref like this:

<SomeCustomReactElement ref={_form => form = _form} />

Thanks for guidance.

[1]: http://airbnb.io/enzyme/docs/api/ReactWrapper/ref.html