First sign is when the tops bend over - wait until the majority of the crop is bent over (some with thicker necks or flower stems may never bend over) and looking pretty dried out. Eat the ones that won't bend over first (must be a joke there somewhere!).
Some people recommend pulling up the crop, washing off the soil and 'curing' them by laying root end towards the sun for a week - best done in a greenhouse where it can't rain.
If you want to store them for any length of time, store somewhere cool and ideally with lots of air circulation (hanging in nets) - a dry, drafty garage or shed is good.