The best-known battery, published by David Eagleman and colleagues in 2007, moved synesthesia testing online and standardized it: repeated color picks scored for consistency, plus speeded tasks where genuine synesthetes behave measurably differently from memorizers.
A battery matters because self-report is friendly to wishful thinking. Plenty of people feel that Wednesday is bluish; far fewer pick the same blue in November that they picked in May, inside the tight radius genuine synesthetes manage.
Scores below a consistency threshold, typically around 1.0 in the battery’s color-distance units, indicate synesthesia. The approach defines the modern standard any casual quiz should be judged against.