It's not at all scientific, but from real world experience you can compare the amount of powder used and it will give you some idea, assuming close to equal barrel lengths and burn rates.
For example, my .204 Ruger and .223 seem pretty close. The 22-250 is seemingly louder and uses 38 grains versus 25-27 for the others. Those are louder than a Hornet that uses 12 grains.
Same thing with pistol. 38 Special is not as loud as a 44 mag, and uses less powder.
There are obviously a lot more factors that go into it, but I kinda use that as a rough idea.