After all the beans I've been picking (there are 6 1-qt. zip bags in the freezer, and that's the stuff we haven't been eating!), I'm starting to get the "feel" for it:
The beans feel wimpy and the surface feels soft until they're ready to pick -- usually "pencil thin" or a little thicker. They have somewhat tougher feel and don't give or bend. The surface color changes too -- a little duller and less satiny.
Pull each one gently upwards until it breaks off the stem, or hold gently with middle, ring, and pinky fingers, resting the top of the bean on last joint of your forefinger, pull gently and push stem off with your thumb.