Use a strong light shining from the back of the work to see if there are any pencil marks from the artist's original sketch. There also may be aspects of a drawing that the artist changed while painting that will show using the light. These are both indications of an original painting.
View the work using a jeweler's loupe, looking for an overlapping dotted pattern of magenta, black, yellow and cyan. This indicates that the work is a print.
Examine the painting for brush strokes.
Look carefully at the back of the canvas checking for uneven or rough threads. This is often a sign of an original painting.
Check for the date on the painting. It is often on the back of the canvass or near the signature of the artist.
Determine if there are numbers on the painting. Numbers, such as 56 of 200, indicate that it is a limited-edition print.
Ask the seller about the provenance (or history of ownership) of the painting, which will prove authenticity of the artwork.