Use a sound level meter or decibel meter
Turn on the meter and select the appropriate weighting, usually A-weighting
Set the response mode, usually slow for steady sounds or fast for changing sounds
Hold the meter at the location where you want to measure sound
Point the microphone toward the sound source
Keep the meter away from your body and reflective surfaces
Read the displayed value in dB
Take multiple readings if needed and record the average
For digital audio, use software meters or plugins that display dB levels
For electrical signals, use a multimeter or oscilloscope with dB conversion
Compare the measured value to the reference level used for that scale