As long as one side of the mains is nominally at "earth ground," you can simply connect only the tip of a suitably rated attenuator (10x or 100x) probe to any point. Use DC coupling to avoid any LF rolloff issues. The line side should show the full line voltage waveform. The neutral side should show the neutral/ground junk, which should be nearly zero in the scale of things. Do NOT connect the ground clip from the probe - it's actually best to remove it from the probe if it clips in, or tape it over if not, to avoid inadvertently making a connection. At low frequencies the probe ground doesn't matter.
As others have mentioned, the line waveform is remarkably distorted, so don't bother comparing DMM, true RMS etc measurements with the time domain scope display. The scope will show what it actually looks like, as accurately as the probe/scope combo can.
Ed
