TrueRTA (www.trueaudio.com) has a spectrum analyzer that has a ocilloscope function, but you will have to reduce the AC line voltage down to less than a volt or else you'll blow your laptop (and possibly yourself) to smitherines. The spectrum analyzer can display the harmonic distortion of the AC line voltage. I would make a resistive voltage devider to reduce the raw AC voltage rather than a transformer or VARIAC since these will introduce their own distortion to the waveform. If you're not comfortable making such a devider, don't attempt it!.

My AC has about 3% total harmonic distortion, and none of this can be eliminated by any power filtering device - these are only made for very high frequency RFI/EMI. The only way to get rid of the distortion (which is essentially harmless by the way) is by power regeneration, which is massive overkill.