Patrick,
Using an indicator is usually the first thing I go for but with the sliding table I’ve noticed that using a straight edge can be more illuminating. This is especially true if the main table isn’t perfectly flat and that will cause you a lot of grief if you’re using an indicator.
Travel a straight edge over the table and eyeball the gap, if it maintains its gap that’s good.
If the main table has a curve in both directions (middle is up) that gap won’t be the same in the center. I would set it so that the center of the travel has the smallest clearance.
More important, in my opinion is that the sliding table outrigger doesn’t change in relative height over the course of travel, so that you aren’t cutting a complex curve into long cuts.
Bumbling forward into the unknown.