Thanks for all of the replies so far. I still don't quite understand how I can tell whether an upgrade is available before I book (the Emirates FAQ says that to do that you need to call the Emirates Contact Centre but one of the answers up above suggested there was a way of telling from the website).

Also I don't know how if you choose the "Auto Upgrade" option under Manage Booking you can know in advance how many miles it would take out of your account to do that upgrade, which seems like an important piece of missing information!