Pinheads appear as tiny white dots along the sides of the bed. They swell into buttons, small round bodies fully wrapped in the volva, the egg-shell skin. The egg stage is a larger, oval body with the volva stretched tight, about to split. Then the stem pushes up, the volva tears and the cap opens.
DMR says harvest before the volva breaks or just after: the button and egg stages. TNAU adds that an opened sporocarp, the technical word for the whole mushroom body, is fibrous and worth less. This is the opposite of oyster, where a fully opened cap is normal.