I have only been through 1 litter, that was last July-Sept. I can't remember at what point during the pregnancy does the female begin producing milk. I'm trying to figure her due date since this was an unplanned pregnancy.
The female may develop a milk line any time after 6 weeks, usually if you can milk her and get milk you are within 7 days of delivery. Over the years I had one or 2 go 10 days but that was the longest.
You can try taking her temp, it doesn't always work but if it drops below 99 degrees and stays there for more than an hour when it goes back up she is in labour. Taking it ahead of her labour a few times will give you a baseline to work from.
Any dark green discharge is an emergency, a placenta has separated and she needs to go to the vet now.
You can run a pregnancy calendar for your girl at www.schaferhund.com. It is very helpful if you know the date of first mating. All bitches are different. Just make sure to supplement her diet, so both mom and pups are healthy.