do you have an image of what you are trying assess similarity for? rectangular building would be easily assessed with minimum area bounding rectangles. You can also accompany that with area and/or perimeter ratios. Shape similarity can be described on a variety of levels
Addendum
as for shape congruence specifically, you can determine interior angles of polygons as well as segment lengths, and hence ratios of those and area, but there are no built-in tools to do so.