How to make a good debugging system for previews
If this is your first contact with Construct 2, you may never dare to venture into a tutorial of Debugging, but this tutorial is just for you, and may help you.
This is not a Tutorial to teach you how to debug code; it is intended to show you a method of debugging your game while in preview mode, and you'll be able to debug anything you want.
Debugging Text Body
First of all, start doing a text object and write some things inside, these things could be an event result, variables, positions, frame rates, booleans, layers, etc.
It's highly recommended to change the debug on purpose, like on the screenshot, where I wanted to test the frame rate while fighting with the Boss, and check if the Boss layer was correct, because 5 min ago I had an issue caused by a mistake, where I created the boss on the wrong layer and the explosion animations were overlapped by the Boss.
Now, you can make the events necessary to show you what you want, and make sure to the debug system only occur on the Preview mode.
I did it this way because on the Options Menu the game will have a secret debug system, where I'll create the debug text on the screen there too.
Including Debug to the Main eventsheet
Testing the debug
Long partnership, debug is your friend!