How to check battery health on iPad Air 2

How do I check the battery health on my iPad Air 2? I know how to see the percentage of battery used by each application. The battery life has been rapidly declining with average use and even when locked and on stand-by. It is nearly 4 years old and used to last close to 2 days, now goes down to 20% before the end of one day and seems to go down twice as fast or more per hour than it should. I have applied all the battery saving tips (from lowering brightness to fetch mail manually, no background refresh on apps, no notifications, etc.) I'm on iOS 11.2.1 - didn't update to 11.4 .1 because of reviews here that indicate some had problems with battery life. Please let me know how to check the battery health myself before I spend time on the phone with support or waiting for a Genius at the Apple Store. Thanks!