Sometimes a mood swing will empower me to make a bold decision that I wouldn't ordinarily make, or at least act on. Most of these are not necessarily imulses, but things that I've wanted or considered over time (years) but never could quite do.
I wait. I'll often fill an online shipping cart, come back to it a few hours later and decide not to buy it.
For big decisions, waiting is the way to go. I wait a month and a half. That way I know I'm not being influenced by hormones or by mood.
I write it out on paper. I summon the left (logical) half of my brain and work it through. I ask myself what I would do if emotion was not involved.
I have a very close friend that I can ask for feedback. My sister, and my SO for sounding boards also.
I am thankful that I am a fairly inhibited person, and these habits give me more of a safety net.
But in general, how do you know you are making a bad decision? You don't... until you regret it. :(
Borderline Personality Disorder
Post Traumatic Stress Disorder