If you open any entity that has an output assigned to it, you will see that there is now an icon containing a dot with an arrow coming out of it located in the bottom left corner of the properties window.

This icon tells us that this entity has an output assigned to it. You can double-click on any specific output to jump to the target entity's Inputs tab.

In bottom-left corner of the func_door's properties, you can see that there is a picture of an arrow pointing to a dot. This signifies that this entity has an input. Again, double-clicking on the input will jump to the triggering entities' properties. Every entity's output creates an input somewhere else. And you can easily see if an entity has an input or output based on these icons.