I'm going to go ahead and post what we are going to measure voltage for in the NEMA 14-30 R (Receptacle), as I have time this morning. Buddy, when you get the multi-tester, and let us know what you get, we'll show you what to dial up to perform this test, and what not to touch on the probes with your bare hands.
The measured voltage (which can actually range somewhat but will probably be in the 110-125 volt and 220-250 volt range) should average between:
Y and X should be 240-Volt,
Y and W 120-Volt,
Y and G 120-volt
X and G 120-volt, ,
X and W 120-Volt,
W and G 0(zero)-Volt
Buddy, there is another reason why I think it's important that you learn the multitester uses, and I will explain it later, as I don't want to overwhelm you starting out. We'll dabble on that later for you, as well as anyone else who may read this in the future. We're just going to keep it simple now.
I also edited the title of your post to clarify for everyone that we are discussing a 240/120 volt split-phase service 30 amp outlet, and not a 120 volt single phase 30 amp one. I will also clarify these differences later too, but they have actually been correctly pointed out by others earlier in this thread.