Feel free to add additional approaches.

Approaches 1 and 2 seem best to me, though neither is without downsides (I'd like someone to suggest something better). 3 and 4 do not seem better and require an additional interface. I don't see any serious problems with approaches 1 and 2. 2 seems cleaner assuming going with it does not cause problems. That most users of the interface do not need the method makes me quite uncomfortable with it though, even though I can't think of concrete issues this will cause us in this case.

CheckableForEmptiness !

Can you motivate that? Clearly the entities could easily implement it, and several other classes could as well. However, that is looking at the interface from the implementers side, rather than the intended users side (which is what I think one needs to do for good interface design). I'd basically be inclined to add such an interface as soon as I'd be able to nicely type hint against it. Does your reasoning conflict with that, or do you think doing an instanceof check is sufficiently better than a method_exists one to warrant the added interface?

I prefer instance of over method_exists when reading code, I have no idea how the php internals actually handle this.

As for speed http://stackoverflow.com/questions/28767294/instanceof-or-method-exist-which-one-should-use says instanceof is marginally faster.

As for having something called CheckableForEmptiness, the use of this in typehints for method params is probably rather limited when thinking directly.
But Item & Property could probably implement this CheckableForEmptiness thing.

But then we may as well just add this thing to EntityDocument and be done with it.

I just wrote this on https://github.com/wmde/WikibaseDataModel/pull/547#issuecomment-134935972:

I can not think of a reason why an entity type should not have an isEmpty method. Basically: Can you think of an entity type where it's not possible to tell if it's empty or not? In a worst case scenario it can simply return false all the time. So +2 from my side, but I want to wait for opinions of other team members.

Any objections to adding isEmpty to EntityDocument? The comments on T110177 and on github don't show any objections. I will close this then as resolved.