I have learnt that keeping them in fridge is an easy solution to maintain their life. Sometimes, its simple tips, that can also work.
There is no guess involved in checking their shelf life. Many websites give this information. I usually share a snapshot for guidance, whenever I share my new perfume purchases here. My goto website is www.checkfresh.com, however, there are others too.
Read some here too:
http://www.checkfresh.com/storing-cosmetics.html