It's surprising how much abuse modern LiPos can take, actually. I managed to fly one into the ground a few years ago, which mushroomed to the point it was nearly t-shaped on one end, and it still worked fine. I didn't actually trust it anymore, of course, but it did work.

I've got a couple of batteries that took on a distinct curved shape after some heavy use, and are still ok. The really key indicator is if the casing begins to swell. A little swelling after long use it unavoidable, but if rectilinear pack becomes a cylindrical one, get rid of it.

pca