Glad to be of help :beer: Adding onto Texun's post, the black probe goes into the terminal marked as COM, and for checking DC volts and most other things, the red probe goes into the terminal marked as V?mA. In order to complete the circuit, the black probe must be grounded and luckily the entire unpainted chassis of the case is grounded by the power supply. So you can either hold the tip of the probe to the case or find a suitable hole or divot to let the probe gently hang from so you can free up the hand for working with the red probe. The setting to use for checking these voltages is 20 under the group marked with a V and a line with three dots under it. This way the best precision is possible for measurement at up to 20v. If you're measuring something under 2v such as a CPU Mosfet or northbridge Mosfet, better precision is with the 2v setting in the same group. For now though, checking the rails just requires the 20 setting. Now, with the red probe you can check the 12v rail by touching it to the terminal of a yellow wire in one of the 4-pin molex connectors for drives or fans etc.. If you don't have any free molexes, you can temporarily unplug a hard drive or fan (while the system is off though 🙂) to use that one or buy/make a Y-adapter for the measurement. When you touch the terminal, you can usually slide the probe in a bit for a more solid connection, but don't push it in all of the way as that can loosen the terminal's ability to grip the male pins. If a solid connection isn't made to the red or black probe, it can give a false or unstable rating, just to keep in mind. Luckily the DMM doesn't draw any power from these measurements so it won't affect the rails that way. For the 5v rail, you can use the red wire in the same molex or you can measure from the 6-pin flat-ish connector that is hardly used. This 6-pin connector luckily has the orange wire which is the 3.3v rail. Usually the probes are too large to stick into the 6-pin connector so you will have to use something small to fit in there that is conductive so you can touch the probe to that for a reading. I like to use a paperclip wrapped in electrical tape with only the one tip exposed so it's insulated in order to handle it while it's live. Also keep the conductive device from touching the case.
Hopefully you can now proficiently measure any rail at any time 🙂 This stuff wasn't in my DMM's manual, so I hope it helps if your DMM doesn't include it either.
