If you have a basic level with no level streaming, there isn’t really a point in checking if stuff is loaded or not because from BeginPlay onward you can be sure everything is loaded and ready (as far as the engine logic is concerned).
If you’re using dynamic level streaming, maybe this screenshot has some clues on how to catch the exact moment a sublevel is fully loaded in.
Either way, if neither is sufficient, we’d need some more details on what exactly you’re trying to do because “all actors in a level” sounds specific, but isn’t specific enough.