You would need a device to measure the intensity of the sound.
Intensity is the power divided by area, it’s similar conceptually to pressure.
Once you have the intensity, you would use an equation to get decibels, which are often a convenient unit for measuring sound.
I0 is the reference intensity, which is often 10^-12 W/m^2. That is the threshold for human hearing.
You can imagine the intensity as being the power of the sound propagating outward from a point like a sphere if the sound spreads equally in all directions. Speakers can also be more directional. The logarithm is base 10 and you multiply by ten in front because of the prefix deci.