how to a/b test turntables

Don't even look and tell the dealer to play one of them at random then the other and see if you can tell.

Be careful, many studies have been done that listen tests have a heavy bias to the more expensive components, and not because they are better. Try to set up a double blind test when you do the audition. For some insights into the bias problem check out Floyd Toole's 'sound reproduction' book. While Floyd deals with speakers, the issues are the same no matter what two components are being evaluated. I also suggest that a third mystery unit be included.

Before serious listening, verify that set up is identical and spot on.

For auditioning, pick out no more than 2 or 3 selections from well recorded albums you know well

For me, it always helps to listen A/B/A

Good luck. Please report back your findings/decision.