How do I connect a C wire to an Utica PEG112CDE steam boiler?
The device labeled "PS-802 ELWCO", is a low water cut-off. The "C" terminal there, is not the C you're looking for.

I believe the white wire from the transformer, is what you'll want to connect your C wire to.  I'm not 100% confident though, since I'm not that familiar with this system.  You may be able to contact the manufacturer for confirmation.