If you look at the datasheet for your chargers, it might tell you what you will see on a voltmeter when it is going through the various charge stages. I use a Victron, - Datasheet - which has fully customizable voltage levels, but the default for AGM is 13.8v at float. That is pretty typical for AGM chargers. During charging, (absorption mode) the voltage is 14.4V on my Victron by default.
And if the batteries sit for a day or so without being plugged into anything (no charger, no loads - FWC kill switch activated) then 12.8 is where AGM batteries settle.
All of this this said, using voltage is a lousy way to determine battery State of Charge (SOC). That's because under load the battery voltage level drops, and it takes time (hours) to "settle". You really need to get some kind of shunt based battery meter. I like the Victron 712 BMV but it is pricey. There are Chinese knock-offs that are supposedly OK too.
