Tennis Pros are often asked how to be more 'proactive' and less 'reactive' when playing doubles.
A quick tip to help your doubles positioning and anticipation is to watch your opponent's racquet head when they are at the net. If my opponent's racquet head drops beneath the net then I can safely guess that the next shot should be a shorter more defensive ball. If their racquet head stays above the height of the net then they will most likely take a more offensive swing.
In short, if their racquet is low, you should step forward. If their racquet is high, then take a step back.