Dial/DTI gauge, magnetic stand, and a big lever.

You don't want to adjust it up to the point there's no measurable play as you risk it binding and no room being left for oil, but you also don't want it flapping about.
When adjusting slides that I can't try moving by hand to test resistance, I usually aim for around 0.02mm of movement on a DTI. On older machines, you may need to allow for worn areas, so you ideally want to adjust things at the least worn part of the travel.