Make sure/check if property is selected in linq .Select() statement with unit test?

Assume I have the following class:

public class Person
{
public string FirstName{get;set;}
public string LastName{get;set;}
public int Age{get;set;}
}

Assume I have this query:

var persons = someListOfPersons.Where(r => r.Age > 18)
                                 .Select(m => new Person(){m.LastName});

As we can see above, I am selecting new Person objects with only the LastName property actually selected. Note that at this point this is still just an IQueryable.

How can I unit test this, to make sure that the LastName property has been included in the select statement?