This depends entirely on the phone/OS. The OS may report only that the battery is charging, and not at what rate. Apps like Ampere rely on the OS to report these things, and if it doesn’t you’ll get garbage numbers. Like the other comment says, get a meter to put between the charger and phone to see what rate it’s currently charging at.
That being said, the charge controller in the phone will likely never draw the max rated charger power, and it will vary with many factors such as CPU load, current battery capacity, temperature, etc.
MEMS have done wonders