A piano must be quite old for the keys to be layered in real ivory, since it has been illegal to use ivory for that purpose since the 1950's. Ivory turns yellow with age and shows hairline cracks much like teeth do.
It depends on what kind of piano you have. Typically the newer pianoes have wooden keys with a finish that makes it look like ivory, being how it's illegal to use ivory in the U.S. If you have an...
There are 88 keys (52 white and 36 black) on a piano, though these days they're made from wood (such as spruce) and covered with a plastic that mimics the feel of ivory.
Plastic and resins are usually smoother than real ivory. Looking at bone under magnification you will see a lot of pitting something that real ivory will not have.
The hot pin test can help you...