It's possible to do this. If the WillEat
method were virtual - otherwise FakeItEasy won't be able to fake it out.
With that change, you could do this:
[TestMethod]
public void EatBanana_CallsWillEat()
{
var fakeMonkey = A.Fake<MyMonkey>();
fakeMonkey.EatBanana(new Banana());
A.CallTo(()=>fakeMonkey.WillEat(A<Banana>._)).MustHaveHappened();
}
I'm still not convinced it's a good idea (as I ranted in the comments) - I think you'd be better off relying on other observable behaviour, but I'm not familiar with your system. If you think this is the best way to go, the example code should work for you.