From another terminal type kill -USR1 $pid, where pid=`pidof dd`. This causes dd to spit out data speeds and amounts transferred so far. you might wrap it up in something like:
pid=`pidof dd`
while [[ -d /proc/$pid ]];
do kill -USR1 $pid && sleep 5;
done
Apparently pidof is deprecated or something?? If you only have one dd process running on your machine you might try pkill -USR1 -n -x dd the problem here being that you will keep asking for the pid of dd, and might get new ones. The -n is for the newest process, -o for the oldest, and leave it out completely to send to all running dd processes, like this:
pkill -USR1 -x dd
enjoy.