A DRO on a CNC mill control panel as shown by the movement on the readout is not a DRO per se........it is indicating the rotation of the ball screw and that is as good as a manual mill with just a thread and nut to show movement.

An actual DRO with a glass or mag scale on a manual mill will indicate actual movement of the table or other slides as opposed to thinking the table moves when in actuality it is the rotation of the screw and the table can be static due to wear or backlash in that case.

However........if you have suspected wear in the Y axis slides you'll need to blue them up and see where they touch.

This is not a perfect indication as one slide in a dovetail will match the other but not in a straight line etc but it will give some indication of where the slide is laying......you need actual dovetail straight edges to test the planes of the faces before the dovetails can be considered a fit......this is done at the manufacturing stage and also in subsequent refitting.

I need hardly point out that if the Y axis slides are iffy, being shorter and so subject to forces from the long X axis table slide, then a refit by a person qualified to wield a scraper etc would be the sane way to go......you cannot expect to work miracles on machinery to new standards without the expertise etc.

The sum of all expectations would be to be able to have reliable moves in the axes with squareness in 3 planes and no wobble etc.

I would think that if you can move the table in 2 planes and get squareness with free travel that is as good as it will get......any slop due to slack slides for easy moves will be a cross to bear, that is a characteristic of dovetails.

I would think that continuous pressurised lubrication "could" possibly make a dovetail slide run on the lube film as opposed to a hit and miss lube where it touches.........big end bearings in an engine are examples of pressurised lube film, but the amount of continuous lube that oozes out of the cracks will make it quite messy.

You cannot have metal to metal contact without seizing up and an oil film does the trick if it is there.

If you can live with .01mm tolerances then dovetail slides will be as good as it gets.
Ian.