There's going to be a lot going on when the United States play Belgium and it's hard to drill the tactical nuances of any game down into a handful of bullet points, but some aspects of the game are more important than others. Here's what you should be keeping an eye on as the match unfolds to figure out who's in better shape and what the USMNT is trying to do against the Red Devils.
Where is Michael Bradley getting the ball?
You can tell a lot about what the United States is trying to accomplish by where Michael Bradley is receiving the ball. Often, Jurgen Klinsmann will use possession as a means of defending, much like Barcelona do. Against certain opponents or in certain situations, the best way for the U.S. to mitigate the attacking threat of their opponents is to make sure they don't lose the ball. The best way to ensure this is to have Bradley drop deep to collect the ball from the central defenders, then carry it forward.
But during this tournament, the United States has been doing something entirely different. They started with him in an advanced midfield role, sitting up high and waiting to get the ball in positions where he can turn and attack the goal, and have moved him into the box-to-box, do-everything position he occupies for Toronto FC. He's capable of playing in all three roles, which is great for Klinsmann, because Bradley's versatility allows him to switch between tactics seamlessly.
Where Bradley's receiving the ball will tell you everything about what Klinsmann wants to accomplish. If he's getting the ball from the center backs a lot, they're defending through possession. If he's playing box-to-box, The U.S. are balancing defense and counter-attacking. If he's close to the strikers, the U.S. is trying to win the possession battle as a means of attacking.
Is Fabian Johnson getting covered?
Right back Fabian Johnson has been a more important attacking player for the United States than any of their midfielders in this tournament. He's been excellent bombing forward and playing crosses into the box, or even shooting on a few occasions. Of course, this is risky, because it means that USMNT turnovers in bad spots lead to opportunities for their opponents to attack down the flank Johnson vacated.
Generally, covering for him is a team job. The right central defender, right midfielder and defensive midfielder have to be on the same page and make sure Johnson's flank is covered without completely abandoning other important areas of the pitch. If it looks like Kyle Beckerman, (probably) Omar Gonzalez and (probably) Alejandro Bedoya are doing a good job of shutting down counter-attacks down Johnson's side, the U.S. is in good shape.
Is Eden Hazard running with the ball?
This is strongly related to the previous point. Hazard is likely to start on the left wing for Belgium and will be directly matched up against Johnson and those covering for Johnson. He's one of the world's best dribblers and is very good at both using that dribbling to set up assists and scoring goals for himself. He's a good player off the ball, but he's much more dangerous as a dribbler. If he's getting space to dribble and time on the ball, the United States are screwed.