diff -r 9169bad392c4 -r db2300032678 tests/quadrilaterals.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/quadrilaterals.py	Mon Jan 22 17:47:06 2018 +0200
@@ -0,0 +1,44 @@
+from math import acos, degrees, radians, pi as π
+from testsuite import warning, error
+from geometry import *
+
+def sign_consistency(container):
+    # Returns whether all elements in container have the same sign
+    return min(container) * max(container) >= 0
+
+def concave_test(model):
+    for quadrilateral in model.quadrilaterals:
+        geometry = transform_to_xy(quadrilateral.geometry)
+        z_scores = [
+            cross_product(v2 - v1, v3 - v1).z
+            for v1, v2, v3 in pairs(geometry.vertices, count = 3)
+        ]
+        if not sign_consistency(z_scores):
+            yield error(quadrilateral, 'concave-error')
+
+def skew_test(model):
+    ''' Test for non-coplanar quadrilaterals. '''
+    for quadrilateral in model.quadrilaterals:
+        for triangles in split_quadrilateral(quadrilateral.geometry):
+            plane_1 = triangle_plane_normal(triangles[0])
+            plane_2 = triangle_plane_normal(triangles[1])
+            skew_angle = vector_angle(plane_1, plane_2, normalized = True)
+            if skew_angle > radians(0.1):
+                yield error(quadrilateral, 'skew-error',
+                    skew_angle = skew_angle,
+                )
+                break
+
+manifest = {
+    'tests': {
+        'skew': skew_test,
+        'concave': concave_test,
+    },
+    'messages': {
+        'skew-error': lambda skew_angle:
+            str.format('skew quadrilateral (plane angle {}°)',
+                '%.2f' % degrees(skew_angle),
+            ),
+        'concave-error': 'concave quadrilateral',
+    },
+}