Upcasting (using (Employee)someInstance) is generally easy as the compiler can tell you at compile time if a type is derived from another.
Downcasting however has to be done at run time generally as the compiler may not always know whether the instance in question is of the type given. C# provides two operators for this - is which tells you if the downcast works, and return true/false. And as which attempts to do the cast and returns the correct type if possible, or null if not.
To test if an employee is a manager:
Employee m = new Manager();
Employee e = new Employee();
if(m is Manager) Console.WriteLine("m is a manager");
if(e is Manager) Console.WriteLine("e is a manager");
You can also use this
Employee someEmployee = e as Manager;
if(someEmployee != null) Console.WriteLine("someEmployee (e) is a manager");
Employee someEmployee = m as Manager;
if(someEmployee != null) Console.WriteLine("someEmployee (m) is a manager");