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.