You would need a Cadex battery testing device to get a truly accurate reading. Unfortunately, these cost thousands of dollars. Depending on the model you get. There are ones for testing and repairing, speed at which it conducts the tests and how many you can do at a time.
Standard battery testers are too unreliable and incapable of telling you what you need to know. They will usually have BAD FAIR GOOD or EXCELLENT. A new battery will consistently never get an excellent reading and a known bad battery could get fair - good readings.
So I would think removal of the battery to test isn't very economical.
Their are several apps ( I think I use battery doctor for my Note II) but they also have it for iPhone.
If the battery is in a phone now and functional, you could try charging it to 100% , putting the phone in airplane mode and put the brightness down to the lowest possible setting. Change the screen time out setting to stay on till button is pressed and see how long it stays on.