Sometimes you can feel her when she's going to slip,but I've never dropped any sand in those occasions. Sometimes she'll be right on the edge,but I've always waited till she slipped ,then shut off,come out with the throttle just to a spot before where she was slipping,drop the reverse a notch or two and keep pulling. Once I knew she was on her feet again,drop some sand.
There was this one spot,a curve on a 2 percent grade where the locomotive would always slip,dry rail or wet. I'd start sand about 50 yards before the curve and went through with no problems.