Mites will not usually kill the snake. However, a snake with a bad infestation of mites will quite probably also have protazoans, mouth rot, lung infections, bacterial infections and such like. Not because the mites cause it but because the snake will be run down and probably neglected.
The best way to diagnose mites early is to look at their eyes. The area around their spectacle gets puffy and whitish if they have mites in this area and in my experience, this is the first area mites will go for.