Moq testing void method

Hi I am new to Moq testing and having hard time to do a simple assertion. I am using an interface

 public interface IAdd
 {
     void add(int a, int b);
 }

Moq for the IAdd interface is:

  Mock<IAdd> mockadd = new Mock<IAdd>();
  mockadd.Setup(x => x.add(It.IsAny<int>(), It.IsAny<int>()).callback((int a, int b) => { a+b;});
  IAdd testing = mockadd.Object;

Since the add method is void, it doesn't return any value to Assert with. How can I assert this setup?