Hi,
One way to test it, is indirectly to the method call, but directly to the method side effect or the class state. For example,
if dummyFunc increments an accessible counter, you can assert the counter value.
Another alternative is to use DoInstead, for example:
int counter = 0;
Isolate.WhenCalled(() => baseObj.callFuncTwice()).DoInstead(context => counter++);
And at the end assert the counter value.
Also we now have a feature : Isolate.Verify.GetTimesCalled.