When you are trying to determine the value of old glass bottles, it's important to remember that appearance is not the only indicator. It takes a discerning eye and knowledge of rare colours, as well as familiarity with popular moulds and designs, to decide whether what you have is vintage or just any old glass bottle.
Compare your bottle's colour with those sought by collectors and antiques dealers. Clear glass has historically been mass produced. Glass bottles in cobalt blue and other jewel tones are more likely to be vintage than those made of clear glass.
Identify the manufacturer if there is a seal on the bottle and determine whether the manufacturer made many or just a few bottles with that particular mould. You may seek out a vintage catalogue or visit an antiques dealer to see whether your bottle is one of a few hundred or one of several thousand. Determining the bottle's manufacturer will help you find out if it's a vintage bottle.
Examine the bottle carefully for distinguishing marks. Vintage bottles often have a handmade look with an uneven surface, known as whittling. Manufacturing techniques weren't always mechanised as they are now. The best way to tell whether your old bottle has vintage distinguishing marks is to examine several bottles of different qualities that are known to be vintage.
Post pictures of your bottle online or take it to a collector's fair to see if there is any interest. Often, you may not know what you really have until someone else shows an interest in it. Examine other bottles and see how they compare with yours. You may find other bottles similar to yours but of an inferior quality. Comparison browsing will not only help you distinguish whether your bottle is vintage, but also help you determine the value of your bottle.