Be sure that your animation cel has peg holes at the bottom or sometimes top of the cel. This indicates that is is an original production cel. Studios have used various peg-hole registrations over the years. Early Disney art will be on 2 peg; later, Disney changed to a 5 peg registration.
Check the outlines of the character. The last Disney hand-inked film was in 1959. This means that the characters were all outlined in different colors. For anything after that, Disney used a photocopying process, which left the outlines of the characters all black.
Take note of the scene. Generally, if there are several characters on a cel in a very good pose, it is likely that you are looking at a publicity cel, not an original production cel.
Watch the cartoon. If you have access to the film that the cel was in, try to find your exact cel. Finding the precise frame will help identify it as original.
Finally, if you are still uncertain, ask a professional. There are many specialized galleries and auction houses that can tell you if what you are buying is original.