Gin is one of the world's oldest liquors, having been distilled from juniper berries in ancient Greece. Italian monks in the 11th century added juniper berries to distilled spirits and the current concoction was born. Gin was particularly popular in England, where at one point it outsold beer by a margin of six to one. Antique gin bottles from the 1800s and 1900s are highly collectable; most common are the "case gin" bottles with four straight sides and a square bottom that easily fit into wooden packing crates. Case gin bottles are typically olive green, with rarer varieties either clear, amber or cobalt blue.
Familiarise yourself with antique gin bottles. Visit the website Antique Bottles and go to the "gin" page for dozens of links and resources. Scour auction sites like eBay as well as speciality auction houses like American Bottle Auctions (see Resources, below). Check out their listings, most of them with photos, and see if you can find your bottle.
Join a club like the Federation of Historical Bottle Collectors (see Resources, below). Subscribe to the group's bimonthly magazine, Bottles and Extras, and check out the directory of bottle collectors. If there's one near you, contact him and see if you can meet, or at least exchange e-mails and develop a relationship. Fellow collectors can almost always either identify your antique gin bottle themselves or steer you to someone who can. All you need is a digital camera to take a picture of your bottle and Internet access to send it.
Pick up a reference book like Kovels' Bottles Price List, by Ralph and Terry Kovel (Random House Reference, 13th edition, 2006). Kovel is one of the most respected names in antique circles, publishing a wide assortment of price guides. This book is a 272-page paperback with more than 350 photos and 12,000 listings.
Contact a bottle appraiser (several are listed on the Antique Bottles website under "appraisals") and send him a detailed description of your antique gin bottle and some digital photos.
Generally speaking, if a bottle has no seams, it may be free blown, which means it was made before 1860. If there are no seams on the bottom, only on the neck, with a shoulder seam that runs completely around the bottle, you have a gin bottle made with a three-piece mould, which was primarily in use from 1840 until 1870. If the side seam runs through the top of the bottle, it was probably made by an automatic bottle machine, which first came into use in 1905 and became widespread by 1920. If your bottle was made in the United States and has a patent number, contact the U.S. Patent and Trademark Office and inquire about a search.