Colour is the start of it. A ripe coffee cherry is deep red — or yellow, on some varieties — but a cluster ripens unevenly, so the picker has to judge each one.
Colour, then feel
A ripe cherry comes away from the branch with a light twist and feels full and slightly soft. If it resists, it is not ready, and it stays for the next pass.
Why we pick in passes
A block is picked several times over a few weeks. It is slower and it costs more, but it is the single biggest thing that separates a clean lot from a mixed one.