You can determine if your backup is complete by looking at the Dashboard tab, which gives you an estimate of how much of your backup has completed and what remains.
The Overview button shows how many items you have already uploaded and how many remain.
The Activity button shows you what files are in line to be encrypted and uploaded to the SpiderOak servers and how much progress has been made on each.
The Actions button lists all actions that SpiderOak is taking: uploading, scanning, syncing, etc.
The Completed button shows you which files have been uploaded to your account.
Note that this information resets each time you close the application, so it only displays files which were uploaded since the last time you opened it.