First introduced in the early years of the 20th century, "carnival" glass earned its name after being given away as booth prizes by travelling carnivals. The bulk of it was made in the years between 1910 and World War II. Reproductions and new shapes were produced in the 1990s as a collector's market for the originals developed. Because it is distinctive in appearance and far from rare, carnival glass is relatively simple to identify and price.
Check that an potential carnival glass piece is genuine by ascertaining how it was made. Carnival glass was press-moulded -- the shapes stamped out under high pressure in a speedy mechanised process. On press-moulded glass, none of the details are sharp-edged because they haven't been worked by hand, while the base typically has a curled rim.
Look next for an iridescent, oily sheen. This is the key feature of carnival glass. Without it, the item is probably another form of press-moulded glassware, like depression or uranium glass.
Find prices by typing "carnival glass" into your search engine. There is no shortage of sites specialising in carnival glass, whether buying and selling or simply gathering information. Use the sites' own search engines to find examples similar to your own. Items of carnival glass are usually described according to their shape (vases, bowls, novelties) and colour (marigold, amethyst, green.)
Find more prices by visiting online auction sites. Go to their carnival glass subcategories and type a description like "amethyst bowl" into the auction's search engine to bring up a list of current lots. Use the site's tracking tool to follow some of these lots through to their conclusion. This will give you a realistic and up-to-date price for a particular piece of carnival glass.
Another way of distinguishing carnival glass from uranium or depression glass is that the latter wares were primarily utilitarian, whereas carnival glass, even when made into cups and saucers, is too frilly and elaborate for everyday use.
Because there are examples of modern reproduction carnival glass on the market, this has had the effect of depressing values on vintage pieces.