how to retrieve fan speed hardware api
とか適当にググったら出てきたぞ

Retrieving CPU/Motherboard temp/voltages/fan speeds etc
https://social.msdn.microsoft.com/Forums/vstudio/en-US/cc34b5e0-dad2-4fed-970c-2218a74dc379/retrieving-cpumotherboard-tempvoltagesfan-speeds-etc?forum=vbgeneral

So researched more and found http://openhardwaremonitor.org/ which has a DLL you can use in your own apps