I have no idea how you would find that out.
I think people do still print stuff if they want a hard copy for whatever reason. I occasionally do.
To see what the print css needs, use print preview, and see what could look better.
I will hide stuff that’s no use in print, like the nav. Re-style things that will use too much ink and just alter whatever looks bad in the preview.
I think browsers generally put that stuff on anyway.