You can run the sensors command on the terminal to check the CPU temperature. It should be installed by default. I tested it on Debian9 and Ubuntu20 and it worked without any error.
$ sensors
BAT1-acpi-0
Adapter: ACPI interface
in0: 12.20 V
coretemp-isa-0000
Adapter: ISA adapter
Package id 0: +75.0°C (high = +84.0°C, crit = +100.0°C)
Core 0: +75.0°C (high = +84.0°C, crit = +100.0°C)
Core 1: +66.0°C (high = +84.0°C, crit = +100.0°C)
Core 2: +70.0°C (high = +84.0°C, crit = +100.0°C)
Core 3: +64.0°C (high = +84.0°C, crit = +100.0°C)
acpitz-acpi-0
Adapter: ACPI interface
temp1: +69.0°C (crit = +127.0°C)
If you want to keep an eye on change in CPU temperature, you can run the following command on the terminal:
watch sensors
