It's very individualized. It's hard to generalize. However, I think a big precipitating factor is increased stress, anxiety, and fear. That's probably the first warning sign for the person subjectively. They might start having strange sensations or ideas in their head. If other people are starting to express concern about their behavior, or things that they are saying, it would be probably wise to take heed. A person might start acting erratically or start to talk about things that don't seem to make sense, or don't seem to be based in reality, but it's not full-blown.