I know what I've read because I read (fiction) almost entirely on my phone and the app shows what books I've opened. From that list I can easily remember if I just looked at is as a possible "next read", started but DNF, or read it through. Since I changed phone last year I'd need to charge and open the app on the previous phone to get some of last year's. It's just a case of going through and doing it. I could probably remember most of them anyway, at least if I finished it. A short DNF might escape me, and the order wouldn't be right.