Precisely determine the actual consumed watt of the device
Precisely determine the actual consumed watt of the device.

Some devices consume a fixed power when on - e.g., a heater, a light bulb, a fan - because the load is constant. Usually you can read the power from the rating label - e.g., 2000 W for a heater. Note that this rating is at the rated voltage so if the voltage changes the power will vary too.
Other devices' power consumption will vary - e.g., a washing machine has motors and heaters which turn on and off at various times, and computers will consume more power at high processor and graphic card loading.
You have two options:

Figure 1. A typical plug-in power meter. Image source: Amazon.

Measure the power consumption using a power meter.
Use the rating label as a guide figure. In the case of a power supply you use the input power for the calculation as that's what you are paying for.