Open the standard Android Phone (Dialer) application
Enter *#*#checkin#*#* on the dialer keypad ( *#*#2432546#*#*)
On the last entry of the *, your phone will pause while it is checking for updates.
Once it has completed, you will see a yellow exclamation icon in the notification (or different icon depending on your installed theme) bar letting you know that checkin has been completed and if an update exists you should be notified.
It also appears that Market updates are also rechecked as part of this process, so any new updated applications will also notify you to upgrade your applications