Hi Neil,
Isolator AAA currently does not have a way to verify that an action was invoked.
Instead why don't you attach a new listener to that action and assert that it was called:
[TestMethod]
public void action_scenario_expected()
{
bool wasCalled = false;
Isolate.Fake.StaticMethods<A>();
A.InteractWithFileSystem += () => wasCalled = true;
A.InteractWithFileSystem();
Assert.IsTrue(wasCalled);
}