Usually #14, which goes with the typical 15 amp fused circuit.
If you insist, you can measure a straight unkinked part of the wire with a micrometer although I myself don't have wire diameter versus gauge handy at this moment.
The average homeowner who lost his house in the Oklahoma tornadoes should move for good and not rebuild. Too much complexity watchdogging the contractor. Too much a chance to be defrauded.