The_Fox - 5:23 am on Oct 5, 2011 (gmt 0)
This is easy if you have the analytics data. Compare your old traffic to new traffic for the www pages. It might be that your old traffic was majorly directed towards the page that you moved, so now they go to the subdomain, provided everything is redirected properly, and hence, the www pages still get the same amount of traffic but it "seems" that they're getting less traffic.
Compare what KWs gave you most traffic earlier. See if they're showing up for the subdomain pages now.
Compare what KWs are getting you traffic for the www pages now and check if those KWs were present in the traffic data earlier.