This test applies to synesthetes whose "cross-wiring" enables them to see and visualize numbers-as-colors. (About 1 in 200 people.)
Scan the square below. Do you discern any particular shape? What is it? (If you have this particular form of synesthesia, the shape should be quickly apparent. If it takes a while to see, you are probably not a "colors-numbers" synesthete.)
In general, the brain may be "predesigned" to assigning "synesthesia-like" qualities (in the form of metaphor) to its sensory surroundings. In the picture below, if asked which shape is called "bouba," and which is "kiki," 98% of people answer the same way.