[xiaoma]

Yeah, it's pretty messy. 被 is usually but not always adverse. And while 被 is the most common, there are at least half a dozen possible passive markers such as 讓,弄,給,叫,據說, etc... The topic comment construction of Chinese further complicates things, since it's very common for passive constructions to lack any passive marker at all.

For example:

The article has been finished. 文章寫好了.

The meeting has been postponed until Monday. 會議延到星期一舉行.