I try to practice mindfulness, so just as a habit, I keep pretty close tabs on my thinking and engage in re-framing and such when I catch myself engaging in distorted or excessively negative thinking. It's such a habit that I do it automatically w/out really thinking too much about it. However, every so often I realize that I'm having to re-frame or counter my thoughts much more frequently than usual. There have even been times when I've wondered if I was engaging in some weird form of self-induced brainwashing. When that starts to happen, it's a big ole red flag that I'm in danger of falling into depression.

Also if I start to notice that I'm not enjoying my kids, perhaps even trying to avoid dealing with them, that's a big red flag as well.