I hear they check 4WD for chains around Los Angeles, but I've never seen it happen in the Sierras.

I've even got through a couple times in my RWD pickup just by holding up four fingers and driving through, though I've also been called on it once.

(And, for the record, I've never been stuck or run into anything when doing this. I save it for when the road is just slushy and putting chains on would put me at serious risk of snapping them and destroying my quarter panels, which has happened once already due to overzealous Caltrans enforcement requiring chains on wet roads.)