Antique is a hard word to define but I, and many others, define it as anything made 100 or more years ago (anything after that is considered vintage).  Of course this is constantly changing.  Identifying any antique glass is simple.  First, see if the glass is marked, finding a mark is the first thing to look for when buying ANY antique, period.   If  you cannot see the mark check the bottom and sides (look closer, use a magnifying glass if necessary).  Many times the mark may be very faint, so use the reflection of the light you are using to aid you.  If you've found a mark, great! You can find the company who made the glass by using a glassmaker marks book.  Or use an internet search engine to look up the mark.  I recommend looking the mark up in a book, it is easier and faster but the internet is a good alternative if you don't have glass identification books laying around.
    
       If you have found a mark of some sort, whether it be a signature, symbol, or trademark just remember: when searching on a search engine you may need to try different combination's of the description you have.  Here is an example: You have found a handled sugar that has the letter "C" inside a diamond shape on the bottom.  You type "glass mark C in diamond" (without quotes) with this particular mark, which is the Cambridge Glass Company's mark, you have found what you need and are satisfied, but finding a mark is not always that simple.  To make a long story short you should always try the advanced search function if your search did not yield results the first time.  If you know boolean operators (some are built into the advanced search function) use them, these can help you greatly narrow your search.

       If you have not found a mark, check for scratches on the bottom surface of the glass this may indicate the glass is "old."  Although this a much less accurate way to assess the age of the glass, it can be useful to someone who has experience in the field.  I you find a piece of glass that has a dip in the bottom that looks highly polished, then it may be valuable as this indicates the glassblower polished the pontil mark.  Polishing the pontil mark is an indication that the glassmaker took an extra step to dress up the glass, but remember, even if the pontil is still intact your piece of glass could still be valuble.

      Some glass, such as pressed glass, can be identified by the pattern.  Since certain patterns were made at different times in the 19th century it can be quite easy to find the exact date of your piece.  Of course reproductions are still made so there is a chance the glass is new or vintage.  A way to tell if your pressed glass is a reproduction is if it is of poor quality. Some indications of this are: low clarity, very dull edges, and light weight.  Of course if you are new to glass you may not notice this at first, but experts in the glass field find these features helpful.  Sometimes it can be very hard to tell whether the glass is a reproduction because some molds of the older companies were reused later, as the glass, once again, became popular.

 A fool-proof way to verify the authenticity of vaseline glass (pictured below) is to use a black-light and shine it on the glass (this is also useful when checking a painting for repairs).  This works only with vaseline glass because a chemical (uranium) was used to colour glass before the WWII era which makes it florescent. 

If you are interested in starting a collection look no further!  You can get some excellent deals on ebay if you know what you are looking at, but be warned, there are some amateurs out there who just use the first description they find.  The safest bet is to go with a seller who has a history of selling antiques.  Although many ways to identify glass cannot be determined by a picture, you always have the option to ask the seller, that's what they're there for!  After about four years in the antiques field I have learned a good bit about the subject.  If you are really interested in antiques these are some books you should look into purchasing: Know Your Antiques - Ralph and Terry Kovel, How Do You Know It's Old? - Harold L. Peterson, The Book of Old Silver - Seymor B. Wyler,  The Kovels' Complete Antique Price List (Latest Edition) - Ralph and Terry Kovel, The Encyclopedia of Furniture (Latest Edition) - Joseph Aronson, The Book of Pottery and Porcelain - Warren Cox, American Glass - George S. and Helen McKearin, and Milk Glass - E. Belknap.  These books are all highly recommended for use in the antiques field and are just the tip of the iceberg for collectors.  You can probably find most of these on eBay!  Imagine that...

Please leave a review on this guide, I hope this answered your questions!