Andrew Mercer
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sudo apt-get install lm-sensors        # or sudo dnf install lm_sensors
sudo sensors-detect                    # one-time probe (accept the defaults)
sensors

coretemp-isa-0000
Package id 0:  +52.0°C  (high = +100.0°C, crit = +100.0°C)
Core 0:        +49.0°C

Watch continuously: watch -n 2 sensors.

Without extra packages

for z in /sys/class/thermal/thermal_zone*; do echo "$(cat $z/type): $(( $(cat $z/temp) / 1000 ))°C"; done

Values in sysfs are in millidegrees Celsius.

Legacy ACPI interfaces

Older kernels and tools:

cat /proc/acpi/thermal_zone/THM*/temperature      # "temperature: 65 C" (often absent on modern kernels)
acpi -t                                            # Thermal 0: ok, 60.0 degrees C

Combine with a load generator and turbostat to see whether the CPU throttles under heat. Sustained temperatures near the crit value mean cooling needs attention (dust, thermal paste, fan curves).