Should get a little darker in color and be fairly easy to shake off the stems, heck a few of mine still have blooms and the rest are with a pretty small berries. As said by cmason the birds know when too!
Yup what Arne said and I have used a large paper clip or a small paring knife to insert and scoop out the pit, holding over a pail cause it's does get juicy.
Good luck and it will make one of your favorite wines!!
What DoctorCAD said, just put them away for at least a year otherwise when they are ready there will be nothing left. Don't ask me how I know!!
They are gone too fast as it is.