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Why Charging Your Gadgets Over 80% Is Such a Bad Idea | iFixit News
(www.ifixit.com)
This is a most excellent place for technology news and articles.
that doesn't answer the question of whether there's a way to tell that their battery is limited to 80% on hardware level, though.
Unless it's lying about the voltage itself, you can be pretty sure it's not limited if it charges to 4.35V. 4.2 is a little more tricky if you don't know for sure whether 4.2 is the full voltage for the cell.