Place the canvas on a table or other flat surface in bright light, and look closely at the brushstrokes. The dimension of the stroke should be in the paint, with the color rising from the surface. It should not appear as a clear stroke rising from the colored canvas.
Examine the nature of the brushstrokes. They should be uneven, applied in different directions, and vary in size -- broad strokes in large areas and very fine strokes in details like eyes, clothing and hair. There should be blotching and raised areas of applied paint. The brushstrokes should follow the direction of the items in the painting, applied in the same direction as the hair flows, for example.
Hold the painting up to the light. You should see variations in the paint coverage on the canvas. Some areas will look almost see-through, while others will be completely opaque. You should be able to see colors shining through in a blotchy manner, rather than crisp lines and even coverage.
Look at the edges of the canvas. If the canvas is framed, peek underneath it. The color underneath should be as dimensional as the visible paint of the artwork. It should not appear flat.