To check your iPhone’s battery capacity, you can follow these steps:

• From your iPhone home screen, open Settings.
• In the Settings menu, scroll down to Battery.
• Tap Battery Health & Charging.
• Here you can see the maximum capacity of your iPhone battery, which is the measure of battery capacity relative to when it was new. A new battery should be at 100%.
• You can also see the peak performance capability of your battery, which is the ability to deliver maximum instantaneous power. If your battery is degraded, you might see a message that your iPhone’s performance is affected.

You can also use some third-party apps or tools to check your iPhone’s battery health and charge cycles, but they might not be as accurate or reliable as the built-in feature.