Backfaces always appear transparent. Is it possible to display them in black or another color? Different from the color of the front faces, of course.
Turn on Face orientation is not an option, as it fills up front faces too.
You can use Geometry node's Backfacing output to mix different shaders. If it's only for preview you can set up different outputs for Cycles and EEVEE. You can set it up so it only works with EEVEE(that is what you get while in LookDev mode), but will not be rendered with Cycles.