The vcgencmd utility allows reading a few dozen hardware measurements on the Raspberry Pi boards.

CPU Temperature is checked by:

vcgencmd measure_temp

Typical temperatures in office environment, with case:

A red thermometer icon GPU-superimposed on the Raspberry Pi display output means the Raspberry Pi is overheating and is throttling the CPU and GPU to avoid self-destruction.

Raspberry Pi 0, 1, 2, 3 temperature thresholds:

CPU temp. [C]	icon	throttle
< 80	none	none
80 - 85		CPU
> 85		CPU & GPU

Raspberry Pi 4 temperature thresholds:

CPU temp. [C]	icon	throttle
80 - 85	none	CPU: 1000 MHz
85 - 90		CPU
> 90		CPU & GPU

The current Raspberry Pi CPU clock speed is obtained from

vcgencmd measure_clock arm

The output is in units of Hertz. The Raspberry Pi CPU clock speed and power consumption is dynamic:

Log temperature measurements with crontab -e. This can periodically log temperature and CPU frequency, e.g. add a line like:

@hourly  vcgencmd measure_temp | /usr/bin/logger

logger writes the measured parameters into the system log.

Related: Monitor Raspberry Pi DC input voltage