USPS tracking has a foolproof way to determine 'early' labeling via 'tracking status'... I just took a screenshot of a few of my Stamps.com entries that just happen to have almost every status category:
Attachment 438573
all are self-explanatory, but the "printed" category is key, it literally means the label has ONLY been printed. Until the status hits 'in transit' or 'delivered', that package label hasn't been scanned.
Now, I've had many packages that have been delivered that never got past the 'printed' status. But if tracking shows 'in transit', the post office has it :)