Find obvious brush strokes. Unlike previous painting styles which tended to minimize the appearance of the brush on the canvas, impressionism often has very pronounced strokes.
Look for bright, unfocused colors. Impressionist paintings are typically very bright, with one color blending into the next and little boundary between the two. This is achieved by painting wet paint over wet paint, a cornerstone of impressionist technique. In previous styles one color had been left to dry before the next was applied.
Check the light level. As with color, impressionist painting are often very bright with a somewhat unfocused light level. Even in the paintings with clear light and dark, the light source is unseen and the dark areas are only really dark in comparison to the light areas.
Observe the subject matter. Impressionism prided itself on its ordinary subjects. Simple paintings of landscapes or people going about their normal business were very common.
Study the painting's arrangement. Impressionist paintings are typically outdoors (e.g., showing either a landscape or a person obviously outside). This became one of the hallmarks of impressionism. Also, since most of the artists went outside to paint, they painted more things that were outside rather than inside.