PS3's aren't known for overheating, most failures are the blu-ray drive, after that the bluetooth not working well enough.
If a PS3 is getting too hot inside the case (outside doesn't matter -- that heat is gone, let it go) you'll hear the fan get much louder than normal.
A typical PS3 has 3 fan speeds:
- wabbit hunting quiet
- I can heaar you
- wow, that's like 360 DVD drive loud
Unless you're hearing the loudest fan speed all the time, your PS3 should be fine.