Now Charles, you know I'm always nice. Maybe a few traces short of a closed circuit,.. but reasonably polite.
You can usually tell if it's a new/minty BC by the sky blue hold down tape that they used. It is a bitch to remove and most people either threw it away or left it on the back of the reel. I don't think that most folks would use it twice so if it is still holding down the tail, it's at least mint. I've bought 15-20 unopened BCs and resorted to using a razor blade to remove the hold down tape and an adhesive remover to get rid of the sticky mess before it got on my reel tables. I've got a couple of them that the previous owner cut off enough of the tape to remove the Dolby calibration. I have to admit, I've thought of doing that myself ( or just erasing them)especially after discussing the use of those tones and found out that the level they are recorded at varies. That extra tape is one of the reasons that I only have a couple that sound worse at the beginning. As for common practice with tapes, I believe that many of the full collections of them had institutional beginnings. Libraries (music departments) classical radio stations etc.. I can imagine that there was a lot of misinformation that was tossed about regarding the proper care of reel to reel tapes at the time. Luckily, it wasn't a very popular format for those kinds of places so they didn't get taken out very often (if ever). I can remember being in libraries with r2r machines built into the listening stations and never ever seeing one used. That was in the '70's when the BC's were current.
So, just be sure to attach leader to the ones that have been shortened. Bad things happen to the first couple of feet of tape no matter how careful you are.