You can find all of your definitions and uses of variables in the View->Variables section.
If you are searching for a Collection, then it falls short there. There is no "definitions and uses" yet for Collections.
However, they're usually pretty easy to trace back.
I would first look in suspect places like OnVisible for a screen. If it is defined before that, then look on the OnStart of the app.
Unfortunately, with this one shortfall of the PowerApps editor, the problem starts back at the person who built the app. If they did wild things like put timers in to populate collections or special conditions that do it, then you are going to spend some time on it.
Not that this helps you, but may help others (as not to perpetuate the sins) - As a good practice, it is ALWAYS good to have at least ONE screen in your app that has documentation and Collection definition functions.
It's easy to throw Collect statements all around, but very difficult to track them back. If you put them all on one screen, you will never have to search again.