Great question about evidence for break in
Hey, Fly. I've had the same question, having re-slung a couple strops w/ new latigo from a local leather shop.
One thing I can promise will be asked is the width. I'm getting to where I don't want another 3" 'cause of the cupping issue. PITA.
On my new leather, I'll strop after a bit of leather treatment, and notice there's still a non-flat surface. So out comes more leather treatment and a coffee cup for it to strop. Give me a lousy edge, will ya. I'll show you!
I can't say I know, but one attribute of readiness has to be flat. Until mine was flat, I could strop and actually have a reduction in edge readiness.
'Will be interesting to hear the old hands weigh in on the question. Thanks for asking.