Wiring – Help with c wire on American standard air handler

There is a set of connections in the air handler that usually includes the "C" terminal, and it should be labelled (though it may be hard to read depending on the location of the control board and/or wiring/etc in the way).

One way to tell for sure is that while the furnace is not running, using a multi-meter you can measure between the "C" and "R" (or Rh or Rc terminals) and you should see 24VAC. R (or Rh and Rc) will already be going to the thermostat.

As an example, on your furnace, from the manual:

The colors may match, but don't count on it. The sure-fire way is to check using a multi-meter as I said above.

According to the schematic, the B terminal does appear to be connected to the transformer.

I'd actually trace the wiring in the unit just to confirm, but based on the schematic it looks good.

You'll want to make sure the transformer has enough power to supply the additional load of the new thermostat, but typically it isn't a problem. If you hook up the new thermostat, and the fuse inside the unit blows. That would be an indicator that the thermostat is drawing too much juice.