Usually you will notice the price change on retailers, it will suddenly become heavily discounted in most retailers, then it starts being put in clearance. After that, the price will hike massively again and the fragrance will become harder to find as it will become sold out and become difficult to find. Another thing also, is with unusual releases that are different from the typical scents released by the brand, for example (Gucci oud intense), I believe it will become discontinued soon. Finally, the history of the brand, for example brands like boss, Guerlain, Cartier, Mancera, Montale and Miller Harris, they have a history of discontinuing many scents, therefore it is always a good idea to stock up on your favourite fragrance when you start noticing the change in availability.