I look for the waiviness in the 54 Series - Devils in particular to indicate originality. WRT to 1937 notes and older, the texture of the paper and as Rick mentioned above - so signs of wear on the corners, edges, discoloration, etc... of the note to signify it was never in circulation.
My conversation with Don and ideas shared with other collectors is that the technology and handling process - by hand - for the older notes made them rarely susceptable to counting flicks and perhaps a light crease. Further, unless the note had been obtained from the centre of the brick and had been pressed from the corresponding pressure would have unlikely escaped some sort of light counting flick.
Having said that, my ideal standars for UNC are without any counting flick at all - and I would definately deine a crease as being more substantial than a flick and in my eyes give a note an EF - AU grade. However, I think where there is a gray area is the original note with a counting flick that from examination one would conclude has seen no wear and circulation. In general I would grade such a note AU-UNC original, and note the counting flick. And with a corresponding price between AU-UNC.
I have - for better or worse - bought notes through major auctions where the note was described as UNC and upong examination had such counting flicks - but felt the note in its sincerest form was uncirculated.
Likewise, the notes that I can truly say were GEM UNC and purchase for GEM unc prices commanded premium dollars - but were certainly GEMs and some of the most pristine and beautiful notes I have both seen and owned.
I guess, what I am also trying to express is that value should also be taken account and noted wrt to originality. And likewise, disclosure, identification of pressed notes, and cooresponding prices.
Hope this hasnt taken the intention of this post in the wrong direction.
BTW, I was sorry to hear about the experience with the dealer. It can certainly take the zing out of what should be a fun time hunting for treasure...