Methods of determining bad lipo cells, what else could i do?

When charged to 4.2v i check the resistance. I also noticed that a lipo left at 4.2v for a day could have dropped a bit in that timespan which i believe is a good way to determine if a pack is going to tank on you 3/4ths the way through its flight (mainly flying 4s). Does this idea seem to be generally accurate or do you see any flaws with it? I'm pretty OCD, just want to see what you guys think.