How do I determine which is my common C wire?
Your C wire is indeed the one that's labeled C
While your setup with the condensate pump is a bit more complicated than most, it doesn't change the fundamental way things work.  Your thermostat still puts 24V on Y to call for cooling, which gets passed on by the condensate pump to the Y1 terminal on your control board.  Your AC compressor is connected to that Y1 terminal, so it sees that signal and starts the compressor; we then know that the other wire coming from the AC is a C wire, and lo and behold, it lands on the terminal labeled &quot;C&quot; on the control board.
As to getting a spare wire, you could use an &quot;add-a-wire&quot; gadget of some sort, convert your furnace to single stage operation, or simply run a fatter thermostat cable, the choice is yours.