Yes you can.
You can doe it by using the Isolate.Verify.NonPublic.WasCalled() method.
here is an example:
//in some class you have this method:
private void Foo(int x, string s)
//in the test code you can do this
Isolate.Verify.NonPublic.WasCalled(fake, "Foo").WithArguments(1, "a");
Please let me know if it helps.