Pseudo classes on elements can be triggered to investigate how an element may react if it were to be hovered over for example. You can right click on a node in the Elements panel and select Force element state. Alternatively, the Toggle element state icon can be clicked on in the Styles sub-pane.
When an element has some sort of state applied, you'll get a little visual indicator to the left of the nodes' opening tag and in some cases the closing tag too (if they are far apart).
We can trigger: active, focus, hover & visited pseudo classes.