Are you referring to someone that is cloaking for a valid reason or someone that you think is using cloaking to better their SE rankings? If its for the SE rankings, I doubt you would be able to tell they were cloaking in most cases (the good ones are extremely hard to detect)
If its someone that you know is cloaking and its for a valid reason, what would you want to do about it? I dont know of any rules that say you cant cloak - just depends on what you serve up
