... it doesn't always need to be a hardware fault, or dust blocking vents. It could just be a software issue which makes your CPU running warm, so the system needs to cool it down.
Go to Applications > Utilities ( a shortcut from Finder is cmd-shift-U) and find Activity Viewer. Then set the menu View > All Processes.
Then keep watching CPU pane with processes sorted from the highest %CPU to the lowest.
The “rogue” processes would appear on the top. Typically, the CPU load of Office programs and usual stuff such as mail is quite low. Safari can be diverse - some web pages use absolute minimum, some (especially those still using Adobe Flash, which is, eventually, being phased out) can be very demanding. Of course, if you're converting a movie or rendering a 3D scene, or play a game, or Photos need to recognise many faces in your pictures, these use a lot of CPU, too. By the way, it is not an error if an app uses more than 100% - every core in your CPU has 100%, and current Macs have from four to sixteen cores.
When I'm in doubt, I run the Activity Viewer on the background and set the View > Dock Icon > Show CPU History, which gives me a quick view if something is going wrong. Again, with just casual use, typically the graphs are mostly black.
The other way, in laptops, is a click on battery icon - it shows Apps Using Significant Energy. But it is not that exact as it shows CPU use accumulated over a long period of time.
Only if the computer runs fans even without CPU load, indeed something could be wrong with the hardware. But you should try these two resets (they only affect hardware, no reset of data or your settings is performed) first:
After that, the best way to identify hardware issues is your visit to Apple Authorised Service Provider which will run full diagnostics.