How do we know a PDF has been updated?


Banded Mongoose
A question was asked over on Reddit that made me think. I checked RPGNow and found an older non-Travller PDF that had an update. I have purchased almost all of my Traveller PDFs from the site here rather than giving another site a cut of the profits.

I know that the PDFs get updated with the errata periodically. How would I know which items have been updated? I looked at the list of downloads I have and there is nothing there to note what the most current version is. I have not seen any emails from Mongoose saying a new version was available for download.
I was checking my library for any updates and noticed that some of my purchases are not showing at all.

Could this be because those items are under revision?