Just curious.
In the absence of really outstanding evidence (the presence of banned or otherwise long-gone prolific posters), I usually get a good 6-12 posts into a zombie thread before I realize what I’m doing.
Then, of course, I can’t just close the thread. I have to scroll until I find out what triggering the reanimation.
Pretty much my experience. Maybe I’ll come back in five years and answer it again.
Normally pretty quickly - as you say, a few threads in. Once I see it is a zombie, I jump to the new entry and try to see what happened. Most are pointless drive-by’s, but occasionally there is a relevant update. Once in a while I find myself engaged without realizing that the thread is a zombie. Frustrating, but nothing huge.
I wish we could figure out a bit more about the drive-by’s. I assume some % are 'bot-generated based on some algorithm set up to find threads like ours, but am not sure. Given how significant the % of internet traffic is supposed to now be computer-based, that is my working assumption.
As soon as I start writing a response to some idiot that has already posted drivel and I discover that their ID is don’t ask I start checking dates.
Come back to this thread in 5 years for my joke.
I make a note of how many thread reads the thing has had, and note the posts/reads ratio. If the latter is a disproportionally high number (from all of the people who have Googled a question or term over the years and then click on the thread), then my suspicions are immediately raised.
Depends on how many “banned” I see under usernames (and of course how high up those posts are, but usually they seem to be within the first 5-6 posts.) Banned posters are usually my first clue, followed by dead posters.
I’ll tend to notice shortly after I add a post without being aware.
ETA: Just checked, this one isn’t a zombie yet.
Sometimes I notice right away, sometimes I will read for a page or two before it finally clicks, or someone makes a zombie joke.
Some older threads seem to have lost the user names on the posts, so that’s one big clue for me.
Sometimes I’ll see a thread title with the check mark next to it showing I responded in the thread, but I don’t remember the thread. Then when I open it I see it was from five or six years ago.
It’s much easier for me to know that somebody is banned than that somebody is dead. The mods don’t change the word underneath the person’s user name to “DEAD.”
Which, all things considered, is probably a good thing.
If its something (a topic) I’m deeply interested in I often don’t catch it until after I post. But a majority of the time I catch it when I read the OP; I’ve gotten in the habit of checking dates. Once I know its a zombie, I’ll usually go to the last post and read back to see if anything interesting/new is going on worthy of comment – or at least a good zombie joke.
When I notice I’ve already posted in it. 
Most of the time there is an obvious reason to check, maybe BANNED posters or references that date the thread. Sometimes my memory will flash a little message that I’ve read this discussion somewhere before and then I’ll take a look at the posting dates. The latter is not always reliable and nowadays I do check dates fairly early into the thread.
Like the OP, I always have to flip to the end to see why it was reanimated.
Sometimes I don’t figure it out at all. In 2010, I nominated a zombie thread from 2003 for Threadspotting without realizing it and it was accepted for the front page.
I have a Greasemonkey script that checks the date that the thread was started, and displays OLD next to the thread title on the Forum ‘home’ page, and another that inserts a big
This thread is X years and Y days old!
before the first and last post of every page in the thread…so if I don’t notice immediately it means I’m ***really ***not paying attention.
Sometimes I’ll see a thread title and think “I’ve got a story about that topic,” only to find that I already posted it 6 years ago. If there are a lot of banned posters, I notice.
This for me also. My third clue is that after I have composed a response in my head, I find that I wrote the exact same response 8 years ago.
Yeah, the BANNED posters tend to be a bit of a clue.
Agreed on the excessive number of banned posters.
Followed by noticing one of my own posts.
Then noticing someone whose name I recognize, but haven’t seen in a long while.
Finally, there’s particularly trainwrecky threads that I was following when they were new so recognize them when they pop up again years later.
I don’t notice that it’s a zombie thread unless someone says so in a post.
Sometimes I don’t until it’s too late. I was really hoping this thread was from 2003.