Another issue with power is that some components may appear to be getting power, but other components don’t appear to be getting power. For example, you can see that your fans are spinning, but none of the other devices appear to be working. In that case, determine where the fan is connected. Is the fan connected to a fan controller on the motherboard or directly to the power supply? If your fan is connected to the power supply and is working properly then that indicates that some part of the power supply is working.
If you’re getting no POST (power on self test), this could indicate issues with either the power supply or a bad motherboard.
Fans have lower voltage requirements, so if something is wrong with the power supply, it may be providing enough voltage to turn on the fans, but not enough power for the motherboard. Use a multimeter or power supply tester to check the voltages being output by the power supply.