Vacuum the vents out. And/Or blow them out with a can of compressed air.
Google 'speedfan for mac' (or CoreTemp)
See if you can get a Mac utility for temps.
If you are running 75 deg C + under load, and have vacuumed, you may need to open the laptop and have a dig around.
Google Mac disassembly or find a .pdf Service and Maintenance Guide.