One of the ways I've checked my turntable speed was with... Guitar Tuner.
I perfectly tuned my guitar and followed solos and chords of few bands I know and figure out that i'm out of tune from original recording by approximately 1/4 of a tone up.
That signalled me that my turntable belt got thinner and longer needing replacement.
Also you can play in unison same digital version of recording through your computer and see which one 'runs' faster. If everything is in-tune and/or runs with same speed than strobe is wrong and needs moneyback or free replacement.
The other suggestion is to realy check if you're using original VPI belt and if not, you'd better order it.
I perfectly tuned my guitar and followed solos and chords of few bands I know and figure out that i'm out of tune from original recording by approximately 1/4 of a tone up.
That signalled me that my turntable belt got thinner and longer needing replacement.
Also you can play in unison same digital version of recording through your computer and see which one 'runs' faster. If everything is in-tune and/or runs with same speed than strobe is wrong and needs moneyback or free replacement.
The other suggestion is to realy check if you're using original VPI belt and if not, you'd better order it.