Something about the tone and the way it fits (or doesn't fit) with the narrative. e.g. a tone which is slightly too jaunty somehow or too contrived. Or too calm given the purported situation of the thread. Thread updates that seem to arrive too neatly, providing a nice narrative that would be a little too neat for real life. Situations that someone wouldn't normally turn to their phone to post on an internet forum in the middle of, with the timing of events a little too conveniently spaced to allow well-constructed updates.
Basically, anything that doesn't quite ring true - where you think "If I was in this situation, would I be doing this?" e.g. in the middle of a series of traumatic events, somehow having the time, presence of mind and desire to get online and start posting. I mean there is a reason why you don't tend to see many people whip out their phone and start writing long updates on it in the middle of a fast-moving and stressful situation, such as their child being diagnosed with a serious disease in real time or the police arriving to check out something odd.
The recent one about the odd email was really entertaining as a piece of narrative, but clearly just not real. You can see why people want it to be, it's like a film playing out in real time so you're waiting for the narrative "payoff". But if you check with your instinct, often on a troll thread you know deep down that "nah, this isn't really very plausible. It's not what people do in situations like this." Equally, watch out for things moving too fast or at odd times in medical, police or legal threads. A troll can do all the research they like on blogs and so on, but if they are attempting to pull off a full "sick child" thread or similar there will always be gaps in the narrative or implausibilities that don't ring true to people who really do have experience in those areas, and which the troll doesn't even know they're getting wrong.
But first, watch out for the tone. Posting tone is difficult, even impossible to fake, so trolls reading this thread aren't really going to benefit, because what gives them away is something a bit indefinable about the rightness or wrongness of the tone which can't really be quantified.