Compare battery life between two (same model) iPhones
Monomeeth provides a good guide on how to do the process of testing your battery manually, but there are two ways to do this automatically (and with fewer variables that could affect results).
Method 1: App
Battery Life (free ad-supported app) lets you check the health of your iPhone's battery immediately after downloading. In my experience, it's very accurate, and reviews seem to agree. Opening it up, you'll see a battery wear percentage on each of the iPhones, and approximate runtimes for talk/browsing/video usage.
The app is a simple way to objectively see if it's time to replace your iPhone battery.

Disclaimer: I have personally used the app but I am not affiliated with its developer in any way.

Method 2: Genius Bar
If you bring your iPhones into the genius bar, a genius can help you run diagnostics on the iPhones to see each battery's wear and current capacity (at no charge).
This is definitely the more official way to do it, but it does require taking a trip to an Apple Store. Read more about Apple's battery diagnostic here.

tl;dr: Download this app or go the genius bar for a battery diagnostic (which is a real thing).

Note: In my personal opinion, whether it's time to replace your battery is more subjective than objective. If the battery life is significantly worse than you remember it being, it's probably time to get a new battery!