[mac336]

I have a marantz pm325 amp that power a pair of floorstanding speakers I have
 
It has a headphone jack and I was wondering if there was any instrument or anything I could do to find out how much power (in watts?) this amp puts into headphones
 
 
looked all over the place online and couldn't find anything

 

[NA Blur]

Short Answer:
Measure the voltage between ground and the negative side on the jack.  Do the math below.
 
From Joule's law we know:
 
#1     Power = Voltage Squared over Resistance
P = V^2 / R
or
 
#2     Power = Current Squared times Resistance
P = I^2*R
 
Since most amps are constant voltage sources use formula #1  ( volume knob controls the constant voltage )
 
Power = V^2 / R
 
Where V is voltage measured by a volt meter on the jack from ground to rail and R is the resistance of the load ( your headphones )
 
Let's say your headphones have a 60 Ohm nominal impedance.  You measure .25 volts out of the jack at 75% volume.
 
Seeing that P = V^2/ R
P = .25 volts ^2 / 60 Ohms
P = .0625 / 60 Watts
P = .001 Watts or 1mW of power
 
See how the power changes depending on the impedance of your headphones?  That is one reason it is important to have a basic understanding of Joule's law.
 
One can work backwards using Ohm's law to find the current draw.
 
We have P = .001 Watts
P can also be equal to I^2 * R
.001 = I^2 * R
We know R is 60 Ohms so we can easily calculate I.
I^2 = P / R
I^2 = .001 / 60
I = ( .001 / 60 ) ^.5
I = .+/- .004 Amps or 4 mA
 
Conclusions:
With a 60 Ohm headphone at 75% volume on our amp we measured a power of 1 mW and a current of 4 mA.
 
From that knowledge you can calculate the estimated volume using the sensitivity of a headphone.
 
The sensitivity of our 60 Ohm headphone is listed to be 106 dB / mW.  Simply multiple 106dB * our 1 mW value found before and we get 106dB of loudness ( pretty darn loud ).
 
Hope that helps.
 
Disclaimer:  There may be mistakes so if you find one please feel free to correct it below.