99.9% of the white metal dentures are a mix of cobalt and steel. They used to be made of stainless steel and nickle but there were too many people with alergies to nickle.

Some of the older denutures can be made of a palladium, silver and gold alloy. With these, if you scratch the metal on a black stone and drop nitric acid onto the surface, the palladium will turn the acid yellow and then it will turn purple. If it contained more silver, it would be red and then turn black under bright light.
 An easy way to check for silver is to use 18k gold tester and rub a strike on the black stone. The 18k gold testing acid will turn the silver into silver chloride and make the metal mark turn white. The lower the silver content is, the greyer the strike will be. Under some cool lights, this color may also look blue. This is due to the copper alloy dissolving into the acid.

I hope this helps,
Charles

ps; contact me if you need me to test them for you