Um, some of these answers are from people who probably havent done this before. No offense but just something to be careful with.
The main indicator is mom. If it seems she is still in pain but it has been more than about 10 minutes since the last puppy, she may have one stuck in the birth canal. THAT is an emergency situation when you should rush to the emergency vet or call your vet. If you do this often enough, you should have your vet's 24 hour contact info.
If she is done, she will expell all of the afterbirth and seem more clam and relaxed and start focusing on the new babies. I also dont let mom eat all of the afterbirth, it causes really thick (and hard to clean) BLACK poop. It gets almost like diarrhea. I usually let her eat about half of it then clear the rest away. Also, you should help pop the sacs and get them breathing. Then work with the umbilical cord. You'll want to use your fingernails and sort of 'scrape' the cord apart. You dont want to cut it with sissors. A ragged cut will help cut down on bleeding.
If you need anything else, let me know and I'd be glad to help!
Good luck and CoNgRaTs!!!!