I don't know what I don't need

Most of my "free" time for the past six weeks or so has been devoted to how this site works, on screen and behind the scenes, rather than what it contains. I've enjoyed it a lot, but now that I am almost ready to roll out the new look, which will be very like the old look, I have an odd problem. What don't I need?

It arises out of my being a cargo cult programmer. That is, I know enough to tinker, but not to invent, and so I tend to take things I find here or there and modify them to suit my own ends. The end result is, obviously, something I am pleased with, but along the way a lot of what I do is to dump stuff that I don't need now and judge I won't need in the future either.

That's what prompted this current round of tinkering, a desire to change to the new default Grav theme and simplify it, and at the same time get rid of some nastiness on small screens that had crept into my site over the past few iterations. A few things are still on my snagging list for the new theme, but there's one I just don't quite know how to tackle.

The original theme came with a bunch of things (mostly SCSS and JS) that I am pretty sure I no longer need, but I cannot be absolutely certain. Is there some non-obvious way to check? Or do I just delete things one at a time and wait for something to break? Where would I even look for clues?