Use a battery utility, such as [http://www.coconut-flavour.com/coconutbattery/|coconutBattery] (for Mac) or [http://www.3u.com/|3uTools] (for Windows). Anything less than 70% of design capacity will require replacement.
You can also use the Battery Health Diagnostic (Settings\Battery\Battery Health (Beta)) in iOS 11.3 and above. It will tell you what the health of the battery is. Once the battery reaches below 80% of design capacity, iOS will throttle performance if the phone experiences an unexpected shutdown.
Once the battery gets to that point (~70-80%), a replacement is the only viable option. You can optimize things by reducing screen intensity etc. but the battery will degrade at a faster rate once it reaches that threshold. Why bother with a hobbled phone when a new battery will give it the zip it deserves.