Overdischarged 18650s and a TP4056 module
Charge them up fully to 4.1v or 4.2v and see if they hold the voltage. Or you could put them in a charge cycle if you have a charger that'll do that (quality RC car chargers work quite excellent for this) for a few rotations and see how they handle that. if there is no issues with the power cycling then I'd say they are just fine.
I've taken some 18650's down to 1.5v (accidently, but also kinda on purpose) but running through a few power cycles they work fine and never had an issue with them.