DG-1000 sail plane pilot reported losing control of the aircraft upon landing after the instructor who was sitting in the rear seat; unaware of the open spoiler position; pulled the spoiler. The reporter suggested that the aircraft should have markings or placards to indicate the position of the spoiler control lever.

Narrative

All of the DG-1000 sailplanes do not have spoiler position labels on the rear seat pilot position. As a result; the spoiler could be a percent open on landing without the rear seat pilot or instructor being aware of its position. Also; on takeoff the rear seat pilot often cannot determine that the spoilers are locked.Event happened to me when an instructor in the rear seat took over controls while landing at 20 feet altitude. I was in the front seat and had spoilers 3/4 deployed. I released the controls to the instructor as he requested HIS AIRCRAFT. Problem; at 10 feet the instructor pulled the spoiler; but they were already open. As a result; the sailplane contacted the runway at a heavy contact on the main wheel but then nosed over as the brake was activated at that spoiler position. The sailplane then swerved sideways because we're just a few degrees not straight; and the sailplane ground looped and slid sideways down the runway and came to a stop without any damage. Lucky. Had the rear seat pilot been aware of the spoiler position by the means of marking or placards the event may not have occurred.Suggest all DG-1000 sailplanes have markings or placards that indicate the position of the spoiler control lever.