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.
Note that you must obey Reference Counting rules at all times. If you call Release on an interface pointer to decrement the reference count to zero, you should not use that pointer again. Occasionally you may need to obtain a weak reference to an object (that is, you may wish to obtain a pointer to one of its interfaces without incrementing the reference count), but it is not acceptable to do this by calling QueryInterface followed by Release. The pointer obtained in such a manner is invalid and should not be used. This more readily becomes apparent when _ATL_DEBUG_INTERFACES is defined, so defining this macro is a useful way of finding reference counting bugs.