Carnival glass is a kind of iridescent glass coated to refract light. It gives the glass a rainbow appearance similar to what you see on an oil slick or the surface of a bubble. The finish of carnival glass pieces can range from almost clear (the iridescence will become visible at an angle) to satiny to metallic.
Carnival glass was first mass-produced in 1908 by the Fenton Art Glass Company in Williamstown, West Virginia. It originally had many different names, such as Iridill and Rainbow Lustre, and remained quite popular until 1918. It wasn't until the 1920s, when it began to be sold at carnivals, that it was dubbed carnival glass.
Vintage carnival glass is seldom marked, so determining the origin of a piece or even distinguishing it from new glass can be quite difficult. Some of the new pieces sold today are reproductions made in re-created molds. Others are reissued pieces formed in the original molds. Some new pieces, like those made by Fenton, will be marked with the name of the manufacturer.
The best way to learn to distinguish the real from the fake is to familiarize yourself with both. Visit glass shows and antiques shops that sell carnival glass. In the meantime, here are some things to look for: New pieces sometimes have an oily feel to them. A coating applied to the mold so pieces can easily be removed causes such a residue. If the pattern is indistinct, an old mold or poor re-creation may have been used. Check for wear on the bottom of the piece. Old pieces will generally show scratches and have a dull base. If a piece is bigger than what your reference books describe, it may have been made from a re-created mold. Familiarize yourself with what new shapes of carnival glass are being manufactured. If in doubt, take the piece to a knowledgeable, reputable antiques dealer.