Look for a crimson color and it should be firm to the touch.

There shouldn't be any soft spots. Squeeze the fruit gently and if it's firm all over it's good to eat. And here's a nifty way to seed a pomegranate - just score the skin lightly with a knife, being careful not to cut into the fruit, put it in a deep bowl of cool water, break it apart gently, and begin teasing out the seeds with your fingertips. The heavy seeds will collect on the bottom of the bowl and all of the pesky white membrane will float, making it easy to remove. Drain the seeds and you're good to go. You won't end up with red-stained fingers either.

Usually by the color. It should be a rich red and free from excessive cracks or splits. As they start to turn, the skin becomes puckered.