How can I spot a perspective inconsistency in a simple drawing?

Treat it as a consistency check, not a test with one exact answer. First, look at edges that represent the same kind of direction in space—for example, the edges going away from you along the box. Ask whether they appear to recede in a compatible way. In linear perspective, parallel lines can appear to converge with distance; railroad tracks are a familiar example. [S1] So if one pair of box edges seems to head toward one area while another similar pair visibly heads somewhere unrelated, that is a clue to inspect.

Next, check the visible faces. Do they seem to belong to one object viewed from one position, or does one face look as if it was drawn from a different viewpoint? You do not need to calculate a vanishing point: simply compare the directions and the apparent amount of each face.

Look for several clues together rather than judging a single slightly wobbly line. Make a small list: which edges are meant to recede in the same direction, which faces are visible, and which edges should meet at the corners. Then compare the drawing with that list.

For example, imagine a box whose top face is visible. If its left and right depth edges seem to recede differently, but the front face and the other depth edges suggest a shared direction, mark those two edges as worth reviewing. A tiny difference from hand-drawing is not automatically a problem. The question is whether the overall set of cues feels like one arrangement.

Yes. Check whether size, placement, and overlap tell a compatible front-to-back story. These are monocular depth cues: relative size, position, and interposition (one thing overlapping another) can help suggest depth. [S1] For a simple drawing, ask: does the part meant to be farther away seem reasonably smaller if the drawing uses that cue? Does its placement fit the intended arrangement? Does an edge that should be hidden appear to pass in front?

These checks support your judgment; they are not a prescribed method for diagnosing a drawing. The source identifies visual depth cues but does not establish a formal procedure or say that every drawing must use them in the same way. [S1] If your style intentionally bends or exaggerates perspective, consistency with that style may matter more than realistic appearance.

Try this pass, without measuring anything:

1. Name the form: “This is meant to be a box viewed from above,” or write your own intended view.
2. Group similar receding edges: Look at the edges that seem to go away in the same direction. Do their directions appear compatible?
3. Compare the faces: Do the visible sides and top or bottom seem to fit the same viewpoint?
4. Check depth clues: Do relative size, placement, and overlaps support the front-to-back order you intended?
5. Mark, don’t immediately redraw: Circle the one or two places where the clues disagree most. Review the drawing as a whole before changing them.

Worked example: suppose a box is meant to show its top and right side. The top is clearly visible, but one depth edge on the right side seems to tilt toward a direction unlike the other depth edges, and a back edge appears to cross in front of the front corner. Those two spots are useful candidates for review. They are clues, not proof: a stylized drawing or a deliberately unusual view may explain them.