Hi Woodie1,
You could test it with a dial indicator (all you guys should have one of these by now, they're available cheap
)
Clamp the ball nut tenderly in a vise, and pull the screw axially, with the indicator against the end of the screw. If it moves, it's not got zero backlash.
Some ballscrews are custom packed with just the right size of ball to eliminate backlash, others have a two piece nut assembly which can be shimmed apart. Most likely you would have noticed this already.
Generally speaking, a zero backlash ballscrew has a firm feel to it, that is, the screw will not rock in the nut.