I don't believe there's a raw command for download completion however here is a little script I wrote for when a download is complete:
on *:FILERCVD:*.*:window -ne @DCC.Status | aline -ph @DCC.Status $adate $asctime(h:nntt) Received $filename from $nick
What this does is tell you (in a seperate window that is minimized) that you received this file (and where it's located)
from this particular person at this certain time. I've altered this somewhat to make it simpler since my original scripting contains other information really not necessary for your purposes.
However if will not say whether the original file the person had is complete.