Read the defense "top to bottom" before the snap. If it's a single safety up top or "1 high", it's probably Cover 1 or Cover 3. If there are two safeties deep or "2 high", it's Cover 2 or Cover 4.
From there, read the corners. Are they in press coverage? If so, they're playing man. Otherwise, they're in zone. If the corners are in zone with 1 high, it's Cover 3. If they're in zone with a 2 high look, it's Cover 4.
Then, read the front seven. How many down linemen are there? Are any linebackers or safeties stunting? What
techniques are the defensive linemen playing?
You have to ask yourself all of this within five seconds, or you're going to get a delay of game penalty. It takes some practice to learn how to read the defense, but it will eventually become second nature. Keep in mind good defenses and users will try to disguise their coverages some times to throw a curveball. That's where post-snap reads come in, but that's for another post.
Once you recognize what the defense is doing, it's up to you to make the right play call to counter what they are doing. One of the popular ways to do this is the
constraint theory. I'll leave it up to you to research what does well against specific defenses.