Although there are mechanisms by which an object can express the functionality it provides statically (before it is instantiated), the fundamental COM mechanism is to use the IUnknown method called QueryInterface.
Every interface is derived from IUnknown, so every interface has an implementation of QueryInterface. Regardless of implementation, this method queries an object using the IID of the interface to which the caller wants a pointer. If the object supports that interface, QueryInterface retrieves a pointer to the interface, while also calling AddRef. Otherwise, it returns the E_NOINTERFACE error code.
See Also QueryInterface: Navigating in an Object in the Win32 SDK