Hard to say without more specifics on your hardware (cetain ports can trigger activity and it depends on how you have them set up), but it should be pretty easy to tell if it is downloading or not. Check the amount downloaded when you go to sleep and then check again when you wake up. If the amount has increased it is working, if not, it stalls out when you are on standby.
Note that when a port triggers activity your comp comes out of standby mode. Logic says if it goes into standby and stays there it is probably not downloading, but logic doesn't always work well with computers.