You can add a new adjustment layer at the top of your composition, with the Levels effect set to the following:
Input Black: 254
Input White: 255
This will have the result of causing any luma values that were below 254 to become black, while anything above it becomes white. Here is an example of it over a fractal noise pattern:
In the export dialogue choose “Master File” of your project first and make sure the effects isn’t happening when watching the video file on your computer.
When the master export is good upload it to YouTube or else video platforms. The file will be transcoded by the streaming platforms to fit all playing needs.
So it appears there is no (built-in) option in FCPX to make separate keyboard short-cuts for separate Effects. Even inside the "Command Editor" mentioned by German Dude, you only have ONE shortcut for applying effects, which is for applying the current Default Effect. But you cannot bind multiple, different effects to different keyboard shortcuts. 😢
I don’t think there’s a way to get zebras on your output, but the Lumetri Scopes will give you a video signal that will show you whether your luma is too high.
Click in the Menu Bar on “Final Cut Pro”. A pull down menu opens, then click on “Commands” then “Customise”
A window will open with all available commands possible. Many won’t have a shortcut and you can set keyboard shortcuts for them. Mind the search bar in the top right corner.
I suggest to duplicate the Default command set before start changing.