Again, we assume that the object is box-like or at least has three obvious families of parallel lines, mutually perpendicular. We'll also agree in advance that oblique projections will be used only for a ``principal oblique view''--the situation where one face is parallel to the viewplane, so that the face is shown undistorted and yet you can see some sides of the object.
First, by looking at images of rectangles, decide whether the view is parallel or perspective.
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Then you may be able to determine the foreshortening ratio and
subtype--
Advice: We are examining a two-dimensional image as lines on paper, and we have to try hard to think of the picture as something two-dimensional rather than three. For example, lines on the paper may not be parallel, but they may represent parallel lines of the object in three dimensions, and so the visual circuitry in our brain tries to regard them as parallel. Useful techniques:
(a) To tell if two lines are parallel, extend them with a ruler.
(b) To tell if two adjacent oblique angles are equal, extend the middle ray across the vertex with a ruler and compare the acute angles you get.
(c) To tell if two line segments are equal, transfer one segment to the other by marking the edge of a piece of paper.