In the hierarchy of hair shirt abilities, I think focusing without any aid is still at the top for me. Guesstimating exposure is OK, working most of the time on neg film, and half of the time on slide film. But guessing focus distance is still hard for me.
So I found just the right technique to do selective focus in that situation: just use a subject that has many possible focus planes. If you aim for one, and miss, focus will fall somewhere else, and still give a perfectly acceptable picture.
The things you learn.