Most managers scout the other team looking at their players and their past games, so they know what they are capable of and how they usually play. (This is obviously just an educated guess, since the other manager can choose another tactic for any given game.) Looking at game shape, which players already played big minutes in previous games that week (for training and game shape purposes), etc. will then give you some extra information to base your decisions on. Of course, you should expect that all experienced managers who play against you will scout you, so they may choose to change their approach due to your strengths and weaknesses.