Well .. u can't actually check overheating ... what u can do is use tools like ATi Tool or Prime 95 and stress out ur system ... and see how long ur system manage to run stable .. and keep the temps monitored the whole time via speedfan etc. ... If u find ur system stable for a very long period of time ... say 10 hrs. at a stretch .. then u need not worry much ....
Also ... better cooling can ensure lower temps. which translate into better stability and longer usage ..... i'd say make sure ur load temps are around 50c for the processor , and around 40c for the rest of the components ....