Use BIOS/UEFI hardware monitor
Use Windows Task Manager with a hardware monitoring app
Use Core Temp
Use HWMonitor
Use HWiNFO
Use MSI Afterburner
Use Mac fan control or iStat Menus on macOS
Use lm-sensors on Linux
Check motherboard manufacturer software
Check CPU temperature in the system tray if supported
Use the command line with supported sensor tools
Compare idle and load temperatures
Check temperatures while gaming or running stress tests