When Rewind is open, the Rewind icon will always be filled in with black. To see what Rewind is capturing at a glance, look at the two dots / indicators next to the Rewind icon in the menubar.
Top dot = Screen Capture
Bottom dot = Audio Capture
If all toggles are all off, you will see transparent / grey dots.
If Screen Capture and Audio Capture toggles are all on, you will see two filled in / black dots.
Click on the Rewind icon to view the dots that match the toggles. This what it'll look like with all toggles on. When the menu is active/clicked, the indicators are tinted to correspond with their respective capture toggles: purple for Screen Capture and orange for Audio Capture. You will notice that the capture toggles in the menu are now color-coded to aide the association between color and the recording state.