Hi Liz,
Most breeders are now breeding Savannah to Savannah at this point. Some still use out crosses for the first and sometime second generation cats for a few reasons. First is that when working on producing F1 kittens, a breeder must choose a mate that the Serval likes, that is large enough and hopefully carry the litter to term. Early on many used Bengals for the black spotting and size. Occasionally a breeder will still use an outcross to bring in a feature that they feel they need to add to their line, and some may be using them because they don't have a Savannah mate.
Generally it is a courtesy to be up front about the parents, but often, it is up to the buyer to ask for photos and/or a copy of the pedigree and to ask questions. You can also tell by the registration code. An "A" cat has an outcross for a parent.
Lori