The studios did not issue certificates of authenticity to each piece that left their buildings. Many certificates of authenticity are merely created by someone with a computer and signing off stating they believe it to be authentic usually without knowing its journey into their hands. As to how you can determine this to be authentic is by looking for the "tells" of what a production drawing or cel should have. First you want to check the paper or cel. Look for the peg holes, paper/cel size, and see if you can determine the paper used (some have water marks).
Second, you want to check for markings near the peg holes. These identify the item as to what show, episode, or other use for the item. The number in the corner is used for its sequence in the scene. These drawings and cels sole purpose was to be used to create the cartoon, not to be kept as art. I believe these items to be authentic, but I do not create certificates at this time.I hope this helps in your decision in purchasing from me.