With my own books, I write: underline, star, ?, or !.
With library books I fold corners; if it's toward the end of a page I fold a bottom corner, top corner for other parts of the page sometimes smaller or larger. I seem to have a decent spatial memory and can find passages on a page quickly. That's one of the drawbacks, IMO, of the Kindle, repagination when paging back ... And no feel for how far I need to turn back. It is easy to mark, though.