// Arrange someSystem.Setup(x => x.MyMethod(It.IsAny(), It.IsAny ())).Returns(Task.CompletedTask); //Act IList newAlerts = em.ReplaceAlerts(alerts, mockedSystem.Object); // Assert someSystem.Verify(x => x.MyMethod(It.IsAny (), It.IsAny ()), Times.Once); 
Unit Tests Moq: Verify Number Times Called
The format for the number of times a moq method is called: 
 
 
 
