First of all, I echo Connie's welcome, Mrs spud. And also, thank you
Della, for your post above
Connie gave a most informative explanation of the use of the various Imperial moulded trademarks. The only thing I would add is that Summit Art Glass (no longer trading) purchased a number of Imperial moulds some decades ago, and made Carnival items that retained a mixture of the LIG and ALIG marks.
I'd like to comment on a couple of things mrs spud said, if I may.
Imperial Carnival isn't particularly rare and hard to find. Some patterns, such as the marigold Fashion punch set, and the marigold Lustre (or Open) Rose bowls are reasonably available and not expensive.
What determines the rarity and value of Imperial Carnival (and indeed any Carnival) is the combination of pattern, shape and colour. Some patterns are hard to find, while others are more available. The same goes for colours - marigold is usually
(not always) more plentiful, while the other colours tend to be more sought after.
It's actually the same with Contemporary Carnival Glass. Some "limited" items are very sought after, while others are plentiful and available.
So how do you figure out what's old, what's new, what's repro and so on? Well there is no easy answer. You need to read, talk to collectors, perhaps go to a Carnival Convention or an auction and chat with other collectors. There really is no substitute for handling the glass and getting to understand its characteristics.
You can post your questions here on the GMB, we'll always try to help. There is also an internet based Carnival Glass club http://www.carnivalglass.org
which has a subscription membership, where you can chat and post photos for ID etc., and there are a number of land based carnival clubs throughout the world too.
Hope this helps a little.