It can be said that all representational painting/drawings, even the most finely detailed and true-to-life, contain some level of abstraction, because they reduce the three-dimensional world to 2D. But it is fairly easy to see if the artist's intention is faithful representation or if the artist has chosen to introduce a perceptible shift toward abstraction. The emphasis moves toward the use of distinct shapes in the indication of form, rather than the full rendering of objects with accurate shading. Complicated shapes might be reduced to a series of smaller or larger planes. Objects might be flattened entirely, or form might be hinted at with minimal shading.
This movement toward the abstract can remove the painting/drawing from being perceived as a photograph and allows the artist's aesthetics to come more into play. Subtle abstract shifts can also be used to accentuate the characteristics of the people and objects found in the scene. Idealization of a subject might occur, or perhaps caricaturization, making the subject appear ridiculous or ugly.
Abstraction of a scene or character might be used to comic effect, to make a disturbing scene more tolerable, or to simply remind the viewer that this is a painting/drawing, an artist's 2D interpretation of something he or she has seen in the real world.
A real world scene might be made almost unrecognizable due to the level of abstraction applied to it, with cubist broken outlines, disappearing edges, and simplified/overlapping forms hinting at multiple perspectives on the scene. Cubist renderings might point to the multi-faceted nature of perception or might be used to express an inability to mentally synthesize the complexity of our reality.
Featured: Tour Eiffel by Robert Delaunay.