The best way to start. This is to go to the Nest compatibility checker. But basically I can tell you that your thermostat can be supported with a nest thermostat or a nest learning thermostat. In all likelihood, the wire that is attached to the B terminal is your Power Common. The nice thing about the Nest installation process, is, it will guide you on how to mark the cables, and move them to the nest.
The AC Cooling Wizard
Just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, I would be happy to keep it open. If there's more we can do, just let me know.