How to check Type belong to in build Dot Net class

Question

I am having class .

Class Employee
{
   int Id {get; set;}
   String Name {get;set;}
   DateTime dob {get;set;}
   Department dept {get;set;}
}

class Department
{
   int Id {get;set;}
   String Name{ get;set;}
} 

I am getting Employee object.

And I want to get property which are not belong to dot net. Here String Name, DateTime dob are dot net class. So i want only Department dept here.

Basically i am traversing the Employee class Property. And want to check each property whether it belong to dot net or not. So how to check that.