I'm doing an experiment about music volume, and the subjects will be listening to the music via headphones connected to a laptop. Is there a way to measure the decibel level of the headphones?
Yes - there are many ways, ranging from accurate, but expensive decibel meters down to free mobile apps. Your challenge will be to try and measure in a way that accurately represents what the ear will hear, so you won't be able to just place the mic against the headphone speaker - you'll want to look at some way of emulating the obstacle to sound waves that a head will provide.
A product such as the 'Soundman Dummy Head' (available from online music gear retailers) might be of use. The product works with special mics that you can place in the ears.
This way you can accurately measure the volume that your test subject will hear during the test and adjust accordingly.
The only downside is this might be a little expensive for a home experiment, the alternative would be to build something similar yourself.
In my experience, If you maintain the output level and source constant, the other major variables affecting sound level would be headphone impedance and material.
So you might be able to calculate a theoretical value based on the producers specifications or measurments and compare that to the data recorded by instruments or that provided by the subject experiencing it.
Doing a bit of research on the topic, it appears that you can even calculate sound pressure and intensity based on the power and distance from speaker.
This site has a nice physics calculator for such values http://www.sengpielaudio.com/calculator-soundvalues.htm