It sounds like you might be too close to the play. Take a step back or two so you can see the whole play. After that, it is a matter of building habits. A great place to start is by officiating the defense. This will allow you to see if the defender has established legal guarding position and evaluate whether contact is a foul or just incidental. As far as watching for violations, by seeing the whole play, you will be able to pick this up. Not a great complete answer here, but a start I think.
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