Synesthesia is not an unknown or entirely uncommon thing, but still. If an artist claims she sells paintings of what she hears, you’d better be certain before you buy it and start boasting to your friends.
Design an experiment to prove synesthesia in an artist who claims so. Try approach it from different levels of anlaysis - bio vs cognitive. Practicing good hypothesis formulation. How to create hypothesis from theory. Testing the hypothesis. Dealing with unsuitable results - is one outlier enough to disprove the hypothesis?