So, you are in the game, and you want to see where to attack quick. There is one trick, that will give you edge on the field. You can visually see control field decay, like on this photo:
Control field starts to decay if one portal is in bad shape, and it gets visually more and more visible. Also fields only decay on the side where the portal is in bad shape. This is great help if you look for a good place to attack, or to defend.