There's currently IsPlayerHolding, but it just returns a bool. There's no way (unless you use the hooks, which isn't 100% that they will pass anyways cause other things may return before you) to know who is carrying the entity.

Perhaps a player-method too, getting what object he is holding?

With holding, I'm associating both physgun, gravgun and Use holding, since you can just do one of those at the same time anyways.

This'd seem like a straightforward thing, source must store this somewhere, I assume.