You've got to watch ES, NQ, YM, and ER2. ER2 is the main contract used to arb. You can sometimes see the spread traders leaning on the ER2 to get long NQ's or ES. Or the opposite. The interplay between the different market centers gives you the overall feel for what the big money is trying to hide. Remember, if you had 100 million dollars of stock you want to buy, you would not just buy it, you would push the futures down so you can get a good price.