After I watched this video https://www.youtube.com/watch?v=8LqTmvuf6tw I bought a tool like that.

While trying to maintain my bicycle I decided to measure the chain using this tool, but how do I know if I should use the 0.75 or 1.0 side of the tool? One side sits kind of like the "new chain" in the video (at 00:58 into the video), while the other sits like the "old chain". Since my chain is old, I'm pretty sure what side I should use, but is it possible to tell this for sure, i.e. what side should be used? Is it written somewhere on the chain itself? On the gears?

Edit: Also, the one in the video has a different range than mine, it is 0.5 to 0.75. How do I know if my tool is right for my chain? What is the unit for this? Inches?