Can you give a few specific examples of threads?
It's hard to tell for sure, and impossible to prove; Astroturf is professionally made and meant to look real!
The ones in general that look suspicious are the one started by newbies, with weird, nonsensical usernames. They obviously don't know much about Bitcoin, yet begin with long, condescending dissertations about why Bitcoin is sure to fail, using arguments already discussed to death in previous threads...
Perhaps, all who care to do it can link here to threads they think might be Astroturf. I'll keep my eye out for specific examples from now on...