How can I determine if a C++ object has been deallocated?

You can't easily tell just by looking at the memory location if the object is still allocated or not. There might be some black magic tricks to do that, but a much cleaner way would be to build a call-back mechanism into the object's destructor.

I think you're asking the wrong question. The question really ought to be:

Why do I feel like I need to look at some address in order to find out whether the pointer pointing there refers to an object or to the garbage left after an ex-object was deleted?

Honestly, you shouldn't be in that situation. If an object is deleted, why are you left with a pointer to its ex-address? Ideally, the pointer should be out of scope. If that isn't possible in your code, it should have been set to NULL.