As mentioned by @Ramhound in the comments, no, there simply isn't a way to know this.
However, it's worth noting that not only selling items will lock you out of this achievement. According to multiple posts in this thread, exchanging currencies counts as a transaction and will prevent you from getting the
For the Hoarder achievement.
You don't need to complete all side missions, just not sell anything at all in the game, including currency exchange.
I did the same thing on my first run, the Canadian currency exchange got me. Can't believe that counts as a sale. I was so focused on getting all the equipment at the Canadian stores that I never even stopped to think that that could be counted as selling.
If you think you may have just accidentally sold an item to a vendor, reload your last save. It seems like the autosave tends to kick in after boss fights and during map transitions, but I can't confirm whether or not completing transactions with vendors will trigger it too.
Thus, to be absolutely certain you don't accidentally lock yourself out of this achievement, the best thing to do would be to create a separate manual save prior to every time you interact with a vendor. That way you'll have a way to backup if you accidentally sell something to them.