How to tell if a cel is real?

How can one authenticate the authenticity of a cel? Are there specific markers or identifiers to look out for? Is it possible to tell if a cel is genuine by merely examining it visually, or are there additional tests or verifications that should be conducted? Are there any common mistakes or misconceptions when it comes to determining the authenticity of a cel? Lastly, how can buyers protect themselves from purchasing counterfeit cels?