In sci.electronics.design Albert wrote:
Does anyone know how to measure or determine the wire size. I have
tons of motors, solenoids, and similar items. I need some #24 wire and
some #30 for a coil, but have to be sure regarding the wire diameter
as it's a critical ap and the author of the article says 'don't
substitute'.

Micrometer.
Vernier caliper may also work.
Either under $30US.