How to determine whether the thermal paste needs re-application?
Notwithstanding what Buscar says - 

If you suspect the paste, change it.
For the sake of a fiver you'd then be certain. 

I actually change the paste on all the graphics cards in the building every year, as part of a full strip &amp; clean; though I only do the CPU coolers if I don't trust the existing application.
On non-Macs, cheap PCs, often the original paste is cheap &amp; inefficient &amp; simply changing it for good stuff can make a world of difference.