Open Settings, then Battery, then Battery Health & Charging. Most people look at the Maximum Capacity percentage and stop there. It is the least useful part of that screen.
Maximum capacity is a rough estimate
It is calculated rather than measured directly, and it drifts. A phone reporting 84 percent can behave far worse than one reporting 79, because capacity says nothing about how well the cell delivers current under load — which is what actually makes a phone stutter or die at 30 percent.
Peak Performance Capability is the real signal
If that section says your iPhone has experienced an unexpected shutdown and performance management has been applied, the battery is already failing under load. That is a replace-now situation regardless of what the percentage says.
The symptoms worth acting on
The phone shuts down suddenly with charge still showing, especially in cold weather.
Charge drops in jumps — 40 percent to 18 in a minute.
The back is warm when idle, or the screen or frame has started to lift.
That last one matters most. A swelling battery is pushing against the screen from behind, and left alone it will crack the display or damage the board underneath. It stops being a battery job and becomes a screen job.
On third-party battery warnings
After any battery replacement that is not done by Apple, iOS may show a service message and can hide the health readout. This is a software lock tied to the original cell, not an indication of a bad part. We tell you before the work whether your model will show it, so there are no surprises when you collect the phone.