Sleep Aid will look at the settings which it knows can stop a Mac from falling to sleep. If Sleep Aid detects any settings that are any other value than it knows, it will present an alert icon to the left and a fix button to the right.
Click on the "Fix" button, and Sleep Aid will attempt to change the setting to what it understands will allow the Mac to fall asleep.
In this list, Sleep Aid includes applications that have asked the system to prevent sleep, it also includes the reason that the application has provided. Sleep Aid will also inspect the application's integrity to try to determine where the application came from.
Click on the options icon to the right of the application name, and Sleep Aid offers the option to switch to that application, or to show it on disk.
At the bottom of the list, Sleep Aid offers the option to display background apps as well as to create a "Test" case, so we can confirm that Sleep Aid is recognizing applications that prevent sleep.