Pinching your dogs' nipples to express a little milk and check to see if it is coming out is not a good idea. When you squeeze the nipple, you are opening her up to mastitis. Mastitis is an infection of the mammary glands. When cows and other animals are "milked," the nipples are treated with a solution to prevent an ascending infection. The lack of a drop of milk at the tip of the nipple will not tell you if there is a problem.
So how can you tell? You have to check the puppies and see if they are getting milk. If they are not, they are not going to nurse, roll over, and take a nap. They are going to be trying to nurse all of the time, and will only rest when they are too weak to try any longer. If the female is not producing any milk at all, the puppies need to be fed a supplement immediately.
You should also weigh the puppies. If they are not gaining weight, she is not producing enough milk. If the female is producing some, but not enough, you need to supplement feed the puppies.