How do i verify an Injected Mock?

I have an issue where I have a Void method (Method A) which I'm passing several variables to, one of which being a list. The method (Method B) which I call the above (Method A), I add something to this list, and pass it to the method (Method A) I wish to test.

However, the class is an InjectedMock.

From trial and error, I've looked at both verifying the injected mock (which throws and error saying it can't do this), and using when() with eq() within it to try and ensure what I'm passing is what I expect. However, this also fails as it's a void.

At a little bit of a loss here!