There can be many, there might be none.
Some examples:
A smell of burning electronics (not conclusive proof of PS failure, but likely to happen).
Smoke
Flickering or resetting video can indicate a PS problem.
Sudden reboots.
Change in or lack of fan sounds.
POST failure.
No response to power button.
Bios reports voltages are more than a small percentage different than 12, 5, 3 volts.
A warning sign
warning
That warning sign said: WARNING! Self destruct on it.
Traffic sign Road sign Warning sign Information Visual symbol
Warning Sign - song - was created in 1985.
A warning SIGN - is a sign telling of dangerous conditions ahead
no
Integers, batteries, dc power supply, ions........
#
yes
no, its a warning sign
That red flag is a warning sign. The cat is warning the puppy.