The custom functions are viewable in the log tab on the right currently:
These have a copy, delete, and edit button.
It would be awesome if there was a fourth button that you could click and it would show you a list of pages / logic blocks where the function is used. A simple way might be to just open a modal with a list. A more elegant way might be to allow you to then click on a given handler where it’s located (sorted by page) and it takes you straight to that logic page on the left.