medicine may or may not have a counter.  If it does not, you can shake the puffer near your ear.  This allows you to listen for the liquid swishing around as well as feel for the liquid moving.  When this kind of inhaler is almost empty, the mist will not be as full as before.  Then it is time to get a new one. You can also count the number of doses you use over time.  This is easier if you take the same amount of puffs every single day. Make sure you get your new inhaler before your old one runs out.  This way you will not miss any doses.