Ask the front rows what they got away with that you missed?
You will learn a lot about scrums that way.
Former Ospreys prop playing for one of the army teams on Wednesday after the game complimented me on my ability to keep control of the scrum, said and I quote, " You never let me get away with any of the things I normally do, fair play mate, however you did allow us to scrum hard which was good"
Happy with that I was!
In response to the OP, Taffy definatly start with massaging their egos a little even the losing team, they may of been some good phases in their game, something along the lines of, " that was a hard game today, you were unlucky to lose but your driving maul was awesome today, one of the best Ive seen this season" Normally gets someone to open up and can lead to furrther discussion about what they can and carnt do legally to stop it, but let them bring that up otheriwse it can look a bit like a lecture.
I think as you stated Taffy only getting 2 assessments in 40 games and having no other way of feedback from the teams then you are well within your rights to ask a few questions after a game, you will of course get varying answers depending on who you speak too!
As Simon T says, Personal Development Plans (PDP's) are great for a ref and allows you to work on things throughout the course of the season but if your stated aim is "to improve my scrum setup" or something similar and no one else watches you except 2 assesors in the season then you may of picked up bad habits, a bit of outside chat can always help and offers a different perspective.