Heatsink temperature probe
If you could measure the temperature at the device/heatsink junction, then you would have an accurate answer, yes.  The further you get away from that point, the lower the temperature reading you will get because the heatsink does not have infinite thermal conductivity internally.

The most accurate measurement you'll make is with a tiny thermocouple buried in the surface of the heatsink directly under where the thermal load is mounted, but that's probably more hassle than you want to go to for a $1 heatsink ;)

You could try putting two big ceramic-case/wirewound resistors on the bottom with a 5mm gap between them, and then the temperature-sensor of your choice (e.g. DS18b20, LM335, etc) between the resistors.  By putting the sensor between the resistors, it will be at pretty much the same temperature as the heatsink/resistor interface.