How do I know if I have the Sortd Gmail add-on but NOT the Chrome Extension?
You will see a Sortd icon in a bar on the right in Gmail that looks like this:
After you have installed the Sortd Chrome Extension, you will no longer see the Sortd icon on the right, but you will see a blue Sortd button on the right of your screen instead. You can use this button to change between the various Sortd view modes - eg. full screen / sidebar / hidden.
Just a note that you can use the Sortd Gmail add-on without the Chrome Extension but the Chrome Extension is much more functional.