The first thing is to check the colour of the liquid that is inside the bottle. Extreme heat and direct sunlight oxidizes the perfume. This causes the water and alcohol to evaporate and the oil to become unstable so the colour of it will be darker, so the chances are it’s gone bad and it’s time to dispose the perfume. But if the color is turning only slightly, the life of the perfume still can be extended: don’t throw it away yet.
Another signal is a sharp change in the aroma. If it doesn’t smell as flowery, musky or spicy as the perfume once did and aroma develops sour or has a vinegar smell, the chances are it’s gone bad and it’s time to dispose the perfume.