Refer to the chart in this sticky:
http://www.buggiesgonewild.com/elect...-readings.html
The specific gravity column is what you are looking for on the hydrometer.
Also, I've learned early on not to just leave the cart plugged in and assume it will charge and cutoff. You should probably take the same lesson. Go back and check it and if it's done, unplug it. If it is not and it has been too long, start troubleshooting.
I'd by the multimeter over the hydrometer if only getting one of them. It can tell you a lot even when you aren't having issues. Another option is to install a volt meter on the cart but that's only going to give you the pack reading, not each battery.