Think about how your opponent is conditioning you to think, and how you are conditioning your opponent to think.
Once you've played enough to do most things by reflex you gain more time to actually try and figure out the person. Like the poster above stated, just try to catch them in some kind of pattern and take advantage of it.
EX: I'm Fox my opponent is Marth, we are about 5 character lengths away from eachother, I charge to test what he will do, he forward smashes, but fortunately for my I had no idea what he would do so I spaced myself to where nothing could hit me but I couldn't hit him either, I react to his fsmash and combo him, while comboing him I pay attention to where his character goes, because of experience I get a good idea of how he is DIing and notice that at the last hit he is holding away from me, so its most likely he will tech away when hits the ground from that hit, so I react to that accordingly and go for another grab.
This is where I judge the skill level of my opponent, based on his reaction to forward smash at that range at the percentage I was at I would assume he is not very good and I would play more aggressive because I would assume he is afraid of me by this point in the match, causing him react slower because he is unsure, because Fox is so fast playing aggressive while he is thinking slowly gives me an even larger advantage.
EX2: I'm Fox and my opponent is Marth, we are about 5 characters lengths away from eachother, I charge and he charges me predicts where I will turn around and then grabs me, he chain throws me and then utilts me, and forward smashes me, with survival DI I manage to firefox and sweetspot the ledge, I waveland off the ledge thinking he will be close enough to grab afterwards, he predicts me though so I miss with my grab and get forward smashed and edgeguarded. In this example the Marth is able to gain control immediately by guessing correctly that I would start off defensive and he caught me off guard because in this example I wasn't expecting that kind of aggression, he was able to keep momentum the entire stock and easily kill me.
This could go many more ways such as him going for the grab me nairing him into my own grab before he can and continuing the pressure for the kill as well as countless other ways. The point is you make calls based on the milliseconds of information you have and keep building a profile, by the second and third games the better player will be winning by more and more.
I have played some people who only attack in one pattern, and all you have to do is hit them at ANY point during this pattern because you know what they will do, after the match they ask what they did wrong and you respond: "you are being way to predictable, mix up your approaches" if you get that kind of comment then you need to really SLOW DOWN your play and just think before you act, if you act the SAME way by reflex every single time, you will be beaten every single time because you are being predictable, it only takes 1 or 2 hits at a high level game to gain enough momentum to finish a stock, just be warned.
