Hi,
You can use a little Physics.
First you weigh the heatsink.
Let say it weighs to 200g.
You then multiply this by the specific heat capacity of the material:
Aluminum 0.897 J/g/degrees C
200 x 0.897 = 179 J / degrees C
You can then heat the heatsink in an oven, or any other way.
Place the heatsink in the orientation that you plan to use it. If you are using a fan, arrange the fan to blow across the heatsink.
Allow the heatsink to cool. While the heatsink is cooling measure and record the temperature changes.
Lets say you raise the heatsink to 60C and the room temperature is 20C
When the temperature has dropped by 63%, (60-20) x 0.64 = 25.6C, Would be 60-25.6 = 34.4C
Record the time, this is the thermal time constant.
Let's say it takes 300 seconds.
Thermal Resistance = Thermal time constant / Thermal mass = 300 / 179 = 1.67 C/W
I apologize for using physics.
Jay_Diddy_B