Is Direct Drive Really Better?

Of all types of TT faults, speed error and wow and flutter are the easiest to evaluate. A strobe disc and a neon bulb will verify that average speed is at least as good as the power line frequency (which is very good). Actually, quartz controlled DD turntables will be better than the line frequency. Test records have tracks with test tones that are designed to make wow and flutter evident. If you can't hear a problem with the test record you will never hear it with musuc.

Tbg...If you want to measure resonant frequency (and I presume Q) there is a straightforward way to do this. You apply vibration to the item you are testing, with frequency swept over the range of interest, and record vibration amplitude of the item under test using one or more accelerometers glued to the item at locations of interest.

You might want to perform such a test during development of the design so as to achieve a non-resonant platter.

Specs...It is obvious that the sonic character of a loudspeaker cannot be defined by specs, although some things like frequency response can be usefully measured. However performance requirements of a TT (not the arm/cartridge) are so simple that I think that specs can tell the whole story.

1..A TT must rotate the LP at exactly correct speed, and without speed variation for reasonable stylus drag force variation.
2..A TT must not generate a magnetic field at the cartridge.
3..A TT must not generate vibration, for example rumble.
4..A TT must attenuate vibration of the base it is mounted on.

All these can be accurately measured. The only issue is to determine what acceptable values might be.

And remember that the real reason for specs is for the manufacturer to verify that each unit he builds has been properly manufactured and assembled so that it performs as well as the design permits. Use of specs to assess the quality of the design is something extra that people do with specs. It is more valid for TT than for most other audio equipment.