Pro tip: watch how a book falls open before you ever touch the spine
I was at the North Bennet Street School book sale in Boston two months ago and an older binder picked up a 1902 dictionary, cracked it at the title page, and told me straight out that you can read a binding's whole history from how it drops. I watched her do it on 12 books in a row and she called the loose signatures on 10 of them before she ever touched the sewing. Since then I check every book that comes into my shop that way first, and I have caught 4 cracked hinges in 6 weeks that I would have missed. If a book flops open wide in the middle, that spine is done and no amount of wheat paste will save it. A tight book resists you a little, that resistance is the whole tell. How do you all test a binding before you quote a repair?
Man, that resistance trick is such a simple tell and I never thought to use it before quoting a job. I feel like a dummy for all the times I missed a cracked hinge until it was too late.