Bakelite is an early plastic (as well as celluloid), but it is very different from celluloid. Bakelite comes in limited colors (yellow, red, black, cream, green) and is heavier than celluloid, making a different sound when dropped on a table top. Also, if you rub bakelite between your fingers and smell, it will have a formeldahyde smell to it. Celluloid will not. Here is a great article on bakelite:
http://reviews.ebay.com/HOW-TO-TEST-FOR-BAKELITE-PLASTIC_W0QQugidZ10000000000099629Celluloid also can be identified by the shank. They typically came in sew through (which doesn't help) or applied shank and ribbon shank. Follow this link (scroll down the page a little more than half way) to help identify celluloid:
http://www.abuttonlady.com/button-workshopVegetable ivory is very easy to tell on the back. Vegetable ivory is actually a nut that was used to imitate real ivory after the ivory ban was put in place. Vegetable ivory buttons will show a clear grain on the back. Grab your jewelry loupe or magnifying glass, turn the button over, hold it in a high light area (under a lamp or in the sun) and inspect the back. You will know it when you see it!
I hope this helps :)
