You can usually see the fans spin, and hear them as they kick in and out or varying it's speed. The problem with notebooks, is that the air vents gets cluttered with dust and fibers, because people tend to leave them in their sofa, chair or something. So over time a layer of dust builds up blocking hot air from escaping. And as the cpu runs hotter and hotter, it clocks itself down to avoid overheating, essentially making it run slower and slower.
If it feels extremely hot when you hold your hand on the chassis around the mousepad area or underneath in the fan area, there could be a problem.
But 36C for each core should be fine, also indicating the fans are working. If it gets over 60C you have a problem. I've seen notebooks with temperatures well over 80C. You could literally fry an egg on them!