How do I address overheating and test fan speed on Mac mini 2018?

I would look into third party software issues which are causing your Mini to work harder than is necessary. You can run the third party app EtreCheck and post the complete report here so we can examine it for possible clues.

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

Of course if you are using resource intensive apps, then the computer will get really hot which is normal when using resource intensive apps.

My opinion is that your Mini likely needs a thorough cleaning due to buildup of dust over the years.

• Always shut down and unplug: Before cleaning, ensure the Mac mini is completely off and disconnected from power. 
• Soft cloth only: Use a soft, lint-free cloth to wipe down the exterior. 
• Avoid liquid contact: Do not spray liquid directly onto the Mac mini. 
• Vacuum for deeper cleaning: If necessary, use a vacuum cleaner with a soft brush attachment to gently remove dust from the vents on the bottom of the Mac mini. 
• Do not open the case: Unless you are a trained technician, do not attempt to open the Mac mini case to clean the inside. 

Yes MacFanControl works fine on a 2018 Mac mini for checking the temp's and the fan speed.

With understanding and caution, it can also be used to manually or automatically increase the fan speed.

https://crystalidea.com/macs-fan-control

Macs Fan Control is excellent.

When you can see the temperature rising you can quickly raise the fan speed. Just remember to put it back to normal (auto) when you have finished.

Never had to use it on my minis but the 2017 iMac could get warm under intensive use.

The normal fan never seems to speed up on any of my Macs but once it gets above 180F I manually speed up.

I have a little app called Hot installed & easy to use up in the top menu bar I can read the temp at all times great.

Also the MacFan control works fine on the Mac mini M4