Many years back my father installed a back yard spigot near his garden.
The shut off valve sits below the frost level covered by a cast iron
sleeve.

This valve hasn't been operated for years. Part of me would like to
begin using it. The other part says forget it as it will likely be too
corroded by now and break instead. Since several weeks still exist
before serious cold arrives what would you suggest?

Another valve exists in the house feeding the line to the back yard.
That line also consists of a spigot on the exterior basement wall of the
house. Past techniques were to shut off the inside valve, open both
exterior spigots, then blow some air through the line to allow expansion
of any remaining water.

Other suggestions?