Many motherboard manufacturers provide a voltage monitor as a feature in their BIOS. Below are a list of common locations for this utility:
The 12v rail should read between 11.8v and 12.4v . It should be stable, with little to no fluctuations. A slight fluctuation of up to .05v is generally fine as long as it happens no more than once every 10-15 seconds. A larger fluctuation, up to .1v can be safe as long as it happens no more than once every 1-2 minutes. A fluctuation higher than .1v, or more often than every 10 seconds could be an indication of an issue with the 12v rail, and you should consider having your power supply tested or replaced.
These readings are taken at an idle state, and may not indicate a more severe issue that occurs when the system is under load. Testing under load with a multimeter is the most accurate way to test a power supply for issues.
Click here to see our FAQ on voltage tolerances