When items are spawned in the Sandbox the underlying systems set's to important variables on all actors. The first is the "Owner" which is set to the player controller who spawned the object (regardless of tool) and the second is the "Instigator" which is set to the currently controlled pawn of the player.
All player controllers are available on the server; whilst on the client only the current player controller is visible (i.e. if you look at any object and the owner is "null" then you're not the owner). If you look at the instigated variable the pawn is always available on the server and every client (until the user disconnects, then this returns "null" or the IsValid() BP Node returns invalid)