How Tested this C# method

I search for this, but i have not found that i want. I have this method, and how can i test?

public class Service{
  private IRepository repository;

  public Service(IRepository repository){
    this.repository = repository;
  }

  public IList<Item> ObtenerItems(int tipo) {
    var spec = new ItemsDelDia(DateTime.Today);
    if (tipo == 0)
      spec &= new ItemsSinPago();
    else
      spec &= new ItemsPagados();
    return this.repository.FindAll(spec.IsSatisfiedBy());
  }
}

I mock Repository for UnitTest, but how i tested spec?

Thanks