Howdy -
Some of us are poor, so we can't afford a fancy oscilloscope. Here's
a link to a pretty nifty software version that works with your sound
card:
http://polly.phys.msu.su/~zeld/oscill.html. I have no
affiliation with this site or the author, I just did a search on the
Web.
Turns out my calibration wasn't too bad!
Mike