Check with a stolen art database;
Ask about the provenance, then actively check it;
Look at the back of works, outside of their frames;
Ask about forensic testing;
Do not buy any antiquity that may have come from conflict zones, like Syria and Iraq
From Michelangelo's 'David' to Van Gogh's 'Starry Night' here are the locations of the world's most famous artistic masterpieces:
Sandro Botticelli, The Birth of Venus (circa 1485/86), Galleria...