The Unicode Ideographic Variation Database defines glyph variants for Han ideographs. For example, Unicode supports two slightly different variants when rendering U+82A6 in Japanese; the font for this test case contains a type 14 ‘cmap’ table that implements them. With a conforming implementation, you should see four glyphs below. In the third glyph (U+82A6 U+E0101), the top stroke below the radical should be attached and slanting up. But in the first, second, and fourth glyph, that stroke should be detached and horizontal. If all four glyphs look the same, or if you see any black boxes in the renderings, it’s a sign that your implementation is broken.