"I don't want to talk about it."
I've found that almost always the most sure sign I need to talk about something is when I really, really, really don't want to talk about it.
When I was a kid, my family went through a number of transitions. Throughout them, my dad was pretty much an asshole. At one point, my mom said to me, "Things that happen in the family need to stay in the family." No, my family was not part of the mafia, but we were in a religious cult, which it turns out is not always that much different. Much later, as an adult, I realized that she had a very different safety concern in her mind when she said that than what I knew as a kid. But, nevertheless, I internalized the lesson that I was never allowed to talk about my dad being an asshole or it would bring shame to the family.