Well the speed of the downloads should be a good clue, wifi can't really touch current 3G speeds. But with android when you see the wifi icon in the notification window that means you are connected and it is system wide the verizon icon should stay but on GSM phones the edge/3G indicator disappears but not the bars. As long as the phone can pull data from the connection no application should be using any of your limited data connection.
The 3G arrows icon light up when it's being used, but I haven't looked to see if it does when using WiFi also.
When I'm using WiFi, the 3G arrows disappear, so that should indicate the WiFi is being used ... now if only I could get the WiFi to work at my workplace ...