A distortion meter will measure distortion levels that would not be
discernable on a scope.

Amounts like 0.05% on a sine wave.

doing it in software. FWIW (probably nuthin' at this point), I
agree with previous posters. By the time you get a sine
wave through a PC, it's probably already got 2,3 or more % distortion
on it.

I remember the analyzer we had, it may well have been an HP. I think,
on its highest setting, it was something like 0.1% full scale, so if
you trusted the linearity of the meter you could measure down to 0.01%
At those levels, everything was very sensitive. All connectors had to
be clean and tight or you'd get erratic/false readings. You could tap
a shielded cable with your finger and make the meter jump.

Good luck!