On the ATX connector, locate one of the orange wires, or if your PSU has that odd Intel plug, just use one of those orange wires. Put you DMM + lead to it and the minus lead to a good ground point. Voilla, your +3.3V rail. Keep in mind that it may be a tad higher than what you read on the motherboard itself under load as the ATX connector imparts a bit of a voltage drop. On your motherboard, a good place to read the +3.3V is the miniature MOSFET(s) used to regulate the Dimm voltage. It(they) often are not as big as the Vcore regulating MOSFETS, so be careful probing the pins if your DMM has relatively fat probe pins. You wouldn't want to short something.
For people wanting to set their Vdimm up near the 3.3V rail value, they should keep in mind that as you increase the Vdimm regulator output to the point where its near the 3.3V input value that feeds it, you will lose the regulating effect since you are asking the regulator to essentially pass the raw 3.3V right on through. Now the 3.3V is already regulated at the PSU, but I suspect having a secondary regulator on the motherboard improves the regulation even better.
Hoot