$ man vcgencmd
...
get_throttled
Returns the throttled state of the system. This is a bit pattern - a bit being set indicates the fol‐
lowing meanings:
Bit Meaning
──── ────────────────────────────────────
0 Under-voltage detected
1 Arm frequency capped
2 Currently throttled
3 Soft temperature limit active
16 Under-voltage has occurred
17 Arm frequency capping has occurred
18 Throttling has occurred
19 Soft temperature limit has occurred