Thermal Resistance of PSU's heat sinks
Measuring the effective heat resistance should be pretty straightforward: you'd affix load resistors to where you'd later put the components that should stay cool. You work in a controlled temperature environment (i.e. indoors at 20°C). 

You put a measurable amount of power through the resistors (e.g. by using a lab voltage supply and measuring the voltage over and current through the resistors). You measure the temperature and wait until you've reached a stable state.

The difference of resistor temperature to ambient divided by the power is your thermal resistance.

But: honestly, I don't think this is worth it. Both the shape and the fact they're fixed right in front of a fan means that they are to be used with forced cooling, i.e. with a fan in a specially shaped enclosure. You'd have to measure in the enclosure and with the fans you're planning to use.