Instructions
Look at the lip or opening of the bottle. Antique bottles had their openings done by hand, so it should not look perfect. It should look flawed and misshapen.
Turn the bottle over and examine the base. Antique bottles should have a mark where the rod that was used to heat and form the bottle was broken off.
Note the color. The darker the brown of the bottle, the likelier it is antique. Dark bottles were commonly used before refrigeration to keep the contents inside from spoiling. A dark brown bottle denotes that it was made for that purpose.
Look at the bottle's stopper, if it has one. If it has a screw top, you can be certain that it doesn't date back any later than 1920.
Look for glass that appears to be hand blown---that is, uneven and imperfect. This denotes that the bottle was not mass-produced, and is probably an antique.
When in doubt, contact an antiques appraiser for his or her thoughts on the item.