Maybe one way is to do a discharge/capacity test every so often. Set up a schedule, on a certain day of the month or every 3 months or whatever. Run a capacity test and internal resistance test and keep a log of it. I have also noticed in times past that when I have either a old cell at the end of its life or I just bought a bad cell, they would charge much quicker than a normal healthy battery. So maybe log the amount of time from depleted to your cut off at 4.1 volts.
Just a few ideas.