var storageManager = new Mock();
storageManager.Setup(e => e.Add(It.IsAny()));
The Add() method expect
You can use Verify.
Examples:
Verify that Add was never called with an UserMetaData with FirstName!= "FirstName1":
storageManager.Verify(e => e.Add(It.Is(d => d.FirstName!="FirstName1")), Times.Never());
Verify that Add was called at least once with an UserMetaData with FirstName== "FirstName1":
storageManager.Verify(e => e.Add(It.Is(d => d.FirstName=="FirstName1")), Times.AtLeastOnce());
Verify that Add was called exactly once with FirstName == "Firstname1" and LastName == "LastName2":
storageManager.Setup(e => e.Add(It.Is(data => data.FirstName == "FirstName1"
&& data.LastName == "LastName2")));
...
storageManager.VerifyAll();