Confirm if your single or double-pane glass has been strengthened. Strengthened glass can include Heat Strengthened Glass, Tempered Glass, and Safety Glass. It is also sometimes referred to as "Toughened Glass." Test single or multiple panes - on double-pane glass, you can test both panes at the same time. This is especially helpful with installed windows when one side of the glass is difficult or impossible to get to. It is also much more efficient in the factory - testing on either side offers substantial time savings and is more straightforward than being required to test both sides. Identify the thickness of the first pane of glass.