One can't necessarily. Sometimes the cops are trolling, and sometimes they are trying to insinuate themselves in to conversation. It really shouldn't matter too much, since @News is not exactly the best place to be talking about anything you wouldn't talk about right in front of a cop, anyway. Don't assume your anonymous posts can't be tracked back to you.
As far as when people call commentors out on being trolls versus being cops, I think normally trolls get called out for asking questions that purposely derail discussion through personal insults ("anarchy fags"), strawmen, or ad hominem attacks. Or by posting as or accusing someone of being someone polarizing ("Megan Guthrie," "Bob Black," etc).
People are most often accused of being cops when they try to use hyper-militant arguments to direct conversation ("they posted a communique about smashing some windows? Real anarchists have their AK's locked and loaded and are ready to shoot some government officials!" - *not a direct quote from @news*), or fishing for information by dropping names, speculating about details that might bait folks to reply with actual factual info that would be privileged, and so forth.
There might be more nuance to it, as mostly I avoid the comments there at this point, except to occasionally inquire about newspaper boxes.