How can I properly check battery problems and health.

So I've been comparing iphone 6's and using some battery health checkers from the app store on them. And I keep getting mixed results. The 'health' rating is changing and also different between different programs...here are a few images showing:

The phones are in the same places but different times and percentages...so how do I check the real health? I'm thinking other apps like timed draining or from a pc? Or do these phones need to be 100 percent charged to show accurate measurements? I can even switch quickly between apps and get different results...Tnx