You would need to use external hardware to accurately measure the battery capacity.
You can use various calibration apps that can estimate through calculation what the battery capacity is but there is no way of knowing how accurate the result is unless you also use hardware testing.
Some phones can be more accurately estimated than others based on whether the manufacturer has allowed direct system reporting of the current. Two manufacturers that have historically not allowed this are Samsung and Motorola. Others may be on a case by case basis.
One app you can try (if you want) to estimate battery capacity is Battery Monitor Widget Pro by 3c. There are other apps as well. Make sure you follow their calibration instructions exactly. Read ahead of time, especially if it involves a purchase, to make sure you are willing to follow their instructions.
Very informative. Always impressive jpr.
I have accepted there is no rational consistency with battery drain, though I am never able to stick to a routine with one battery.
One or more spares is the way to go. I was thinking of portable power banks, but you are still charging the same battery and stressing the port.
That app may work, but I'm not too fussed to get paranoid now.
If I had just spent 700 - 800 on say a new Note 4, and hadn't got a penny/dime for an expensive OEM battery, I would study those apps in detail if the battery was poor.