Just watch him play piano.
https://www.google.com/search?q=slim+gillard+plays+pianp+with+back+of+hands&rlz=1C1VDKB_enGB1018GB1018&oq=slim+gillard+plays+pianp+with+back+of+hands&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIJCAEQIRgKGKABMgkIAhAhGAoYoAEyCQgDECEYChigAdIBCjI2NTM1ajBqMTWoAgCwAgA&sourceid=chrome&ie=UTF-8#fpstate=ive&vld=cid:85eeceeb,vid:v51Q9h505Cc,st:0