heat, how hot is hot

I inherited this Mac from my daughter. From the time I got it, it had a slight fan or HD sound. Nor knowing were either are placed, I assumed it was the fan since it would come on after a short period of working time.

This sound has stopped, and both speaker areas feel a bit warmer than before. I have no idea how warm it can become without a fan. So my question is, how do I determine if the fans need to be replaced and or how to I test the Mac to determine if thee fans are indeed bad.

Thanks

Peter