Professionals check for visual imperfections, incorrect materials (using magnets for coins), and the specific "feel" of the paper or canvas.
The most common reference is the Oscar-winning Austrian-German film directed by Stefan Ruzowitzky. It tells the true story of , a secret Nazi plan during WWII to destabilize the British economy by flooding it with forged banknotes. Los Falsificadores
The Castle of the Counterfeiters is a title associated with the famous anime franchise. Professionals check for visual imperfections