Product review (5 stars): Easy, foolproof warning if you freezer being without power thawed your food. Set and forget.
This is great.  I have been using a cup with water that after it froze, I put a quarter on top.  In case the power goes out and the freezer has issues the quarter dropping to the bottom will clue me in.

This is a much nicer option.  It ties to one of the shelf openings and after you allow the water to freeze you add the ball and screw on the top.  If you have a problem the ball will sink to the bottom and if it freezes you can easily tell that you had a problem because the ball will be stuck at the bottom.  Great way to tell you food thawed.

Just add water to the fill line, let it freeze, add the ball and screw the top.  Done.