This tip works on both iPhones and Android smartphones (and probably on any other phone, smart or otherwise, that contains a battery). From Nick T on battery problems you can see:
If the battery of your phone is removable, simply take it out with caution (after turning the phone off, of course) and look for symptoms like bulging, corrosion near the metal terminals, and green or white-ish stains. These are all signs that the cell is about to kick the bucket. If you don’t see anything wrong with it, proceed to the next tip. If you find suspicious stains or if your cell has developed a hump, however, it is a good idea to ask your carrier or vendor for advice as your battery most likely needs to be replaced. Don’t put the old cell back inside the phone as you don’t want it leaking any nasty chemicals; these may damage the phone’s circuitry. Instead, seal the battery in a plastic zip bag and make sure you recycle it once it is confirmed to be faulty by a professional.
What about problems you can’t see?
Not all phones have batteries that can be easily inspected by the user. If that’s the case with your handset, you can diagnose the health of its cell by monitoring how fast its charge level drops. It is not supposed to drop by two or more percentage points at a time. (Most phones allow you to have their battery level displayed as a percentage in the status bar. If you can’t find the option in its settings menu, try using a widget.) And if your battery goes from full to zero in a matter of hours even when you barely use your phone, its is probably a goner.
My iPhone 6 Plus can go about three days on a charge, a bit less if I play games or use FaceTime.