Have the batter stand one bat length from a barrier (I prefer a net to prevent damage to the bat, but you can use a fence). Have her take her normal swing. If she hits the barrier, she is unlocking her elbows before her shoulders and getting wide on the swing.
Rear Barrier Drill:Place a barrier directly behind the batter and have her take her normal swing. If the bat hits the barrier, she is dropping her hands. You can use this barrier even when you are using a pitching machine or live pitching for instant feedback to the batter that she is dropping her hands.