Using a killawatt hooked up to a 12v 2a ac/dc converter, ran through a boost converter up to 15v, into a morningstar sg4, i noticed after discharging a healthy 12v 18ah agm overnight during a power failure (%50 soc), the killawatt showed ~10w being used. As it charged this wattage droppped, down to 500mw measured by the kill a watt. Basically, this shows the pwm switching of the charger as it tops off the battery to full capacity uses less and less current. Eventyally dropping down to almost nothing. The 12v 2a adapter not connected to a load plugged into the killawatt shows 0w consumption.
You can get a current sensor for an arduino, or killawatt for a ac/dc converter, and using a pwm charger, it will show you how much watttage is being pushed into the battery. As it approaches 0w, you know its charged fully. But this is with a sg4 smart pwm charger. Might not work with less advanced chargers that are only 2/3 stage, or just using a wall wart. Worth a try though imho if you want to by a killawatt or one of those current sensing sensors for an arduino instead.
I'm going to do this with a 12v unknown ah (prob 100ah+ new) that was badly overdischarged, 10.7v when i found out it was shorting in the car it was attached to. The battery was new, and only in there for ~10mo. I think there was a short that discharged it pretty bad, but perhaps it can be desulfated (white film on plates).
Just a thought.