That's on the individual players - if you put up a formation which is, for example, crowded on the left-side, the 'i' button at the top of the screen next to the 'Cancel' and 'Save' buttons will turn into a red '!' button. If you click on it, then it'll tell you that the formation is concentrated too much on the left flank, and the opposition can easily win battles on other parts of the pitch.
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