The potted horseradish plant that I failed to do anything with last
year (except for watering it) is still alive (new leaves). I'm
pleased, of course.

When is the right time to pull it up? (AIUI, the right way to harvest
it is to pull the whole thing up, bring the biggest bit of root into
the kitchen, and replant the smaller bits.)

Is it possible to tell from what's above ground whether the root is
worth pulling up (rather than leaving another year)?