The best test for most of us is to simply compare the gems side by side. If you have two diamonds of the same grade and one is significantly brighter than the other, the cut is the difference. Please understand that the above discussion assumes we are talking about round diamonds. Because of their symmetrical proportions, all the major facets can be cut at the same angle. The same does not hold true for other shapes.
Many people prefer a marquis shape.
This is fine, but do not expect a marquis, or any other shape, to be as brilliant as a round. On a marquis it is necessary to cut a number of facets to accommodate the shape. The angles these facets get cut at vary, slightly to greatly, from those that give the greatest brilliance. These are gems that only have eight facets running from the girdle down and eight up to the table. That makes a total of 17 facets.