You can buy almost all of them from
www.comc.com and be assured that they scan for fakes and attempt to determine trimming/recoloring during their inprocessing. They also accept returns on any card you find questionable within a certain time period.
Dealers' common boxes are not going to have many, if any, fakes. And if you have any question as to their authenticity while at their booth, you should feel comfortable asking them. To my knowledge, there just aren't people faking 1956 commons, even high numbers. The cardstock would be difficult to replicate, they're an abnormal size, and there's no guarantee of a return on investment.
Stars, sure. I would be slightly leery of, but again, I hear more about 1952 reprints and 1959 fakes than '56.