All you have to do is look at the changing orders and market depth before and after the trading window opens and closes, on almost every stock, every day, for the past x years...
JP Morgan simply got caught because they were stupid enough not to realise that the derivatives system IDs every trader with respect to the placing of ever order (and not just the execution of the order), and their traders were making "fake" buy/sell orders in the order of USD $10-100 million on a daily basis.
