Warning: SPOILERS for The Haunting of Bly Manor.
There's a subtle way to tell if a character in The Haunting of Bly Manor has been possessed by a ghost - at least willingly. The second season in Netflix's Haunting anthology series, The Haunting of Bly Manor tells a very different story to that of The Haunting of Hill House. Sure, both shows take place in haunted mansions filled with ghosts and dark pasts, but that's where the bulk of the similarities stop. Rather than the ghosts trying to harm the guests or the house trying to keep the families there forever, Bly Manor's haunting comes from one person, specifically.
It seems practically every ghost in Bly Manor was "created" by the Lady in the Lake, who killed several people over the centuries and trapped them on the grounds - though there may have been others who were merely trapped there because they died in or around the manor, like Hannah Grose. While those ghosts typically remain in the background and are generally harmless to the people living there, because they themselves were victims of the Lady in the Lake, some ghosts - Peter Quint and Rebecca Jessel - figured out how to possess people indefinitely. And when they do so, there's a way for the viewers to tell.
As evidenced by Peter possessing Miles and the Lady in the Lake possessing Dani, when people willingly accept ghosts into themselves, they develop heterochromia: a condition in which people have two different color eyes/irises. It's a subtle yet intriguing way to let audiences in on the secrecy of possessions. Plus, it's not a way for people to go back and tell which scenes Miles willingly let Peter possess him in but also when Rebecca didn't possess Flora.
There's no explanation for why The Haunting of Bly Manor's characters develop heterochromia when a ghost possesses someone with that person's permission but apparently not when the ghost forces themselves in. Judging by the show's themes as well as the fact that possession is a fight over the person's body, the heterochromia can be logically explained by the ghost overtaking the person's soul. At that point, it's not one person in the driver's seat, but rather two drivers having control of the body. One day, one of them may have full control which is why Dani chose to go back to Bly Manor and drown herself in the lake.
Again, The Haunting of Bly Manor's ghosts are notably different from the ghosts of Hill House, because the mythology surrounding them has been explored more profoundly and there's a sense that their existence has consequences for themselves, not just for the living; they forget who they were and start to fade, and there are rules to what they can and can't do. All of this was particularly evident with Peter Quint, Hannah Grose, and unfortunately, the Lady in the Lake living inside Dani Clayton.