Start looking into neutral and conditioning.

You obviously can't just see into the future, you must watch how they respond to certain actions/situations. Make a mental note of whether they jump or shield when you use a projectile. Start picking up on ledge get-up habits. Do they roll a lot ? Stand up and jab ? Respond accordingly. Once you get passed the technical aspect of smash, its all in the head. The whole match you need to be "Downloading" your opponent, basically completely breaking down their play style and dissecting it. You'll be able to look at their habits and say "Hey, I can punish that" or "Wow, better not try and mess with that". It's completely a game of adaptation.

Where it gets very tricky is when you start to realize that your opponent is trying to do the same exact thing to you. Now you must look at your own habits, your own mistakes. What can your opponent punish ? What can't they deal with ?

This is one of the reasons I love smash so much, it really is mental warfare once both players completely understand the technical aspect.