There’s a few options for something like that on the market, I’ll attach a link for one of the ones I’ve used but generally speaking testing the amperage on charging isn’t the most reliable method for testing the battery as there’s several things that can effect that. With iPhones specifically there’s an app you can use for your computer called CoconutBattery that will tell you the exact health of your battery and I’d recommend that for this situation. Here’s an amperage meter I’ve used in the past and however I’d look around for other meters that might work better for you because there’s a lot of options out there.
https://www.adafruit.com/product/4232?gclid=Cj0KCQjwwuD7BRDBARIsAK_5YhVoiZqLaOavQ7LKyDVHaaT59pW-MtTOWgI7gNU-83R7cq9MNH7PPeMaAsd_EALw_wcB