How to measure the RF output power of the PCB board?
The most correct way to measure RF power is with an RF power meter. These come in various shapes and sizes from vendors like Keysight, Rohde &amp; Schwaz, etc.
The tricky part here is that you might not have a clean, intended output point to measure from (if your antenna is integrated on the same PCB).

These devices need to be calibrated in order to make the measurement.

Alternatively, you can use a device like a spectrum analyzer to measure the output power. This will however not give you quite the same accuracy, and can also not be calibrated as easily to take into account cable losses. However, at 800-something MHz that you are working at, cable losses will not be too significant (it all depends on quite how accurate you need to measure this).

If you need to measure the power while the device is running, you can use power meter IC's. These are common as they are used for RSSI measurements, gain control, etc. Some out there are able to measure to sub-0.1 dB accuracy.