You can almost always follow the opponent's DI in up-air. You watch the spin axis if their hurtbox is below the spin axis (ideal) they will pop out along the axis. If they are above they will pop out at 90 degree angle to the spin axis.
You visually track the impact sparks from the mult-hits and follow them with your stick. This info is in the combo thread, but maybe not prominently/clearly enough.
It's a critical skill, maybe the most important for the character. Even the low combo window characters like villager/ness cannot fall out, they're stuck.
I think up-air doesn't even have an SDI modifier in the code. I think it's the highest level of inescapable in mult-hits. F-air + up-smash is 0.8, and really should be 0.5 (or zero). Correct me if I'm wrong data experts.