They are easily spotted in the sand by their eyes poking up from where they burrowed themselves. You don't have to worry about them spotting you, as they are completely blind. What you have to worry about is sound. If you strike an enemy or object near them, they will leap out of hiding and spray their gas in that direction.
Note that they will hear your footsteps if you get within striking range and leap out anyway.
- ↑ Dark Souls II Collector's Edition Guide description.