Originally Posted by whoamI
Welcome to the board. The short answer is that there's no real way of knowing. Even age is not an good indication. I've had a 200ml bottle of Guerlain Vetiver EDC for 18 years and even know that there's about 10ml left it's still pretty much fine.
The two things that really deteriorate fragrances are air and light. With these factors in mind, your chances of getting a good bottle are excellent if the bottle is an atomizer as opposed to a splash bottle, but the most important factor is that the bottle is boxed. There's a lot less likelihood if it is that if will have been exposed to light and, hence, more likelihood that it's fine no matter what its age. This matter more when buying from a store which has their older bottles out in the open as opposed to e-retailers who frequently sell unboxed testers and unboxed older bottles, but have the good sense and practice to keep them well stored and away from light. Don't necessarily discount a bottle because it's not boxed, but by all means it's good practice to box your own bottles over the long haul.
Most designer fragrances tend to last longer than niche fragrances (and even within the group of niche fragrances there is great variation. Annick Goutal fragrances for example don't last as long as most other niche fragrances. Creeds are notorious for poor quality control and deterioration. This has more to do with their crappy cheap spray mechanisms, which leak and which, by extension, let air in, than it does with Creed's "fabled" use of natural ingredients). The newer a designer and even niche fragrance is, the less likelihood that you'll will have problems with deterioration because of the use of preservatives and even light blockers like benzophenone and more modern, effective bottling technology.
I've been buying fragrances for over 25 years now and the "bad bottle" is the exception to the rule. The one exception to that rule is Creed fragrances. Even their large "flacon" bottles leak, which means they're not air tight, so when buying Creed, caution is definitely the best policy. With designer fragrances, older Chanels have a greater tendencey to go off than most other designer fragrances. This is not the case with their newer (post 2000) formulations.
A practical suggestion: if you're really worried about a particular store and its bottles, ask the sales assistant to let you sample first before buying. Many will let you do this, especially if they're a specialty store and don't want to lose your custom. If they don't let you test before you buy, buy the bottle, open the package in the store, and spray it right then and there in front of them. If it's "off", tell them you think the fragrance is "off" and you want a refund or at the very least an acceptable bottle. Your chances are much better if you do it right on the spot than returning a week later.
Having said all this might make it sound like there's a potential bad bottle lurking around each corner. That usually isn't the case and most retailers will take back bad bottles, as will most e-retailers and ebayers. Basenoters will be glad to let you know who you can trust and who you can't. Don't stress too much over this and enjoy the quest as much as the arrival at your destination.
I hope all this helps rather than overwhelms.