As usual, matthew is steering you in the wrong direction.

The ONLY way to tell if the little kittens are truly nursing is immediate exploratory surgery.

You can pick up a scalpel and suture kit at any medical supply store, preferably a Veterinary
medical supply store.  You want to make sure you know where to make the incision, so I would
purchase a Veterinary Surgery for Dummies book at any bookstore, preferably a local bookstore - I
don't like to support the big Borders/B&N type chain stores.

Once you have studied the anatomy of a kitten (THOROUGHLY!), you can proceed to make your incision.
 Don't worry! I know what you're thinking: "It'll hurt the poor kitten!" But contrary to what the
Christian Conservatives will tell you, animals don't feel ANYthing for the first week or so of life.
 Obviously you don't want to disfigure the poor little things, so be careful!  Once you have
penetrated the stomach, you can examine its contents for the presence of milk.

Now, be careful!  Just because one kitten does or doesn't have milk in their stomach, that does not
mean that the others will be the same.  To be sure, you should repeat the procedure for each kitten.
 It may seem like a lot of work, but if you are truly concerned, it is the least you can do.

I can tell you are a devoted Cat-Lover!  Good luck!  Your Love will insure your kittens' survival.

--Cadman