How to determine whether the thermal paste needs re-application?

How can I determine whether the heat sink is no longer firmly heat-tied to the CPU?

This appears to be the case for a seven year old MacBook, but I'd like to confirm without going to the trouble of re-applying the thermal paste just to find out that the old CPU (a core 2 duo) will indeed trigger the fan even for simple tasks.

I am looking for a recipe such as: Running a given process on a MacBook with a core 2 duo in a room with ambient temperature of 20C will not trigger the fans for at least 10 minutes.

In retrospect it's solvable. I can now run various jobs on each new MacBook and determine which processes trigger the fan. The same recipes should continue to work for the lifetime of a machine. If you have a core 2 duo and you can describe a specific testing recipe, please do.