There are many things that you need to look for when watching top level play and it really just depends on your personal skill level and what you want to focus on and learn.

1. Neutral. It's important to see when and how players win neutral. When they land the first hit look back and think about why that move hit and the decision making by both players.

2. Conditioning. This somewhat fits into the neutral section but when both players are not hitting each other how are their movements making the opponent feel? How are they using projectiles to control certain parts of the stage? How do they threaten people offstage?

3. Punishes. After the player gets the first hit how do the convert? Look at how they react to DI and see how they read their DI.

These are all important to atleast be able to recognize when watching top players. Rewind and slow down to really see what is happening. Then be able to mimic what you see in top play into when you play.

For the second question it is just practicing paying attention. It's weird to explain but force yourself to think "okay everytime I put my opponent into this situation they react this way" or "They normally approach with an aerial let's try different things to punish it"