Strangely, I can find out that information on just about any airline in the world using legitimate sources, and last time I checked, I wasn't a hacker.
A few years back, when we were first starting our segregation of services to other major hubs from EWR (ORD, ATL, DFW) to Terminal A, we would send some young, agent-in-training over to the DL, AA, and UA boarding gates to count the pax boarding each flight to the aforementioned destinations. I'm not sure if we still do this, I cannot imagine that we continue to do so in this age of cost cutting. In addition, I have to believe there are some other ways of obtaining this information. This is clearly an example of "spying" on other carriers, but I would in no way call this underhanded or predatory, but simply just a competitive measure to see how well the opposition is doing.