GD,
While that is a cheap version of an Oscilloscope, even with the suggested additional parts to look at the wave shape, you can't use it to do either a true THD (Total Harmonic Distortion) measurement. All you can do is look at it an see if it is pretty.
This came up recently when an acquaintance could not run his new inverter microwave from the CCV portable generator. We got things together and found that his machine produced a really big 3rd and fifth order that fooled the power supply of the microwave.
The microwave ran fine with the power from my 48yo Onan even with the distortion cause be cylinders firing.
Matt