To determine if somebody has been a “genuine” synesthete post mortum is very difficult if not possible. Synesthetic expression through art or lyrics give insight about the multisensory perception of somebody, but may not scientifically be accurate. Therefore it makes more sense to ask those who are alive. For example Collin Pharell, musician and outspoken synesthete. Here a few insights in his synesthetic everyday life.
Meet The Famous Musicians With Synaesthesia, A Condition That Means You Hear Colours | NME.COM