Most have decent eyesight (barring blind snakes!), and do rely on sight for hunting. Some species even have slit pupils like a cat and have reasonably good night vision. But they can't focus their eyes as efficiently as we can, so their vision is somewhat limited compared to ours, hence why they rely more on chemosensory. If you approach a snake, chances are it can see you, but it'll probably flick its tongue to gather more information than it can by sight!