That would be the key: making some kind of test rig so you could rate/group your cells by capacity.
It's the same issue anyone runs up against if they're using used batteries in a conversion, like the ForkenSwift. Used is fine, as long as your weakest battery meets your range target. But if you're getting the dregs in terms of old batteries, it's nice to be able to test before you go to the trouble of putting it in the car.
FYI, here's one DIY way to capacity test old 12v batteries that conceptually could be applied to lithium cells:
Easy Test of Battery Amp-Hours Capacity