Almost everyone has an "old bottle" somewhere. The term antique is somewhat subjective and a bottle doesn't even have to be antique to be collectible. What collectors look for is bottles that were hand made. The date that I use is pre-1915. Around that time, the manufactures began making bottles by machine. The better the machines became, the bottles became more consistent and lost alot of the unique characteristics that make them collectible. PLus they could be pumped out in mass quantities.
To determine hand made, you have to check the seam of the bottle. Generally the bottle was molded in halfs. You will find a line of glass seam running up the side of the bottle and, if it was hand made, the seam will stop about an inch from the top and it looks like the lip was spun or tooled by hand. On a machine made bottle the seam is much finer and will go clear over the top lip. Plus, hand made bottles are usually all unique. They have lots of flaws, bubbles, color variations and are generally crude. That's what makes them so special.
Plus , the older bottes used to Emboss the name and content in the glass. It makes them identifiable and provides a way to give them a pretty exact date in time.
One of the fun aspects of old glass is that clear bottles that are pre-1915 turn amethyst when exposed to the sun's rays. The chemical in the glass that was used to make them clear, reacts to ultra-violet rays. Some turn quickly in a year and some take years. They all turn different.
None of the other colored bottles turn, only the clear. (ok, some milk glass will go pink.)
The machine made bottles generally went to an applied label. Plus the pure food and drug act eliminated the outrageous cure-all entrepreneurs and paved the way for mostly clear glass so you could see what you were getting. Of course there are still many collectible newer bottles. The factors of shape, color and embossing still apply to machine made bottles.