Setting an oscilloscope to read a noise level
Think about the problem.  You want to see a small AC signal superimposed on a much larger DC voltage.

The obvious answer is AC coupling.  That will block the average DC voltage, showing you short term variations from that voltage.  With AC coupling and 10 mV / division, the noise should be about 3 divisions tall, although there could be bigger peaks.

One problem with this method is that you are basically high-pass filtering the signal, but the high pass rolloff frequency is chosen for you by the scope.  Sometimes that's not optimal.  For example, if you are looking for 100 kHz switching noise on the 12 V signal, but it also has some 120 Hz power line ripple, then that ripple may overwhelm the noise you are looking for.  In such cases, make your own high pass filter.  Put a cap in series with the signal, then a resistor to ground that is significantly lower than the scope probe resistance.  That way your capacitor and resistor dictate the high pass rolloff frequency, so you can place it where you want by using different capacitors or resistors.  In this example, 2.2 nF and 10 k&Omega; would work well.  That puts the high pass rolloff frequency at about 7 kHz, which will attenuate 60 Hz by over 100 while leaving 100 kHz essentially untouched.