Oh boy, you are going to love your Newf!!!
I'm sure you will get all kinds of information from other more knowledgeable people than I, but here are some of the basics.
Genetic Hip and joint problems are major concerns for this breed as well as genetic heart disease. The breeder should show that the parents have been either PennHIP or OFA'd. Here is a site that explains the two:
http://www.workingdogs.com/ofa_penn.htm. Also a guarantee against genetic heart disease.
Of course the pup must be up to date on shots and worming.
A detailed application should be given to you asking about the household, fencing, why you want a Newf, experience with large breeds, etc. If the breeder is close there should be no hesitation on letting you have an on site visit. Full disclosure on Sire and Dam should be given, Pedigrees, registrations, the ability to register your pup.
You should have to disclose your purpose for the dog; pet or show. With exact criteria for both. Pets should require a signed statement that you will spay or nueter and if they haven't done so already, micro-chip.
The breeder should be available to answer any and all questions that may arise after receiving your puppy.
And to me, this is the most important:You should have to sign an agreement that if for any reason, no matter how long you have the dog, one month or 5 years, you must give the dog back to the breeder for rehoming.
Hope this helps a little. I'm afraid your biggest problem will be picking your pup once you've started looking. How can you choose? You're going to fall in love with every single one.
Good luck future Newf owner.