Testing an async function that may not callback

I have a function like this

helloWorldAsync(cb)

This function will return the callback if there's an error.

var spy = sinon.spy()

helloWorldAsync(spy)

expect(spy).to.have.been.called

However this code is running asynchronously so I believe the runtime (is that you call it?) is passing right over the helloWorldAsync() function and expecting spy to have been called early.

How do I monitor this function's callback?