Hindsight is 20/20, as they say, so when you act on a terrible idea it seems obvious after the fact. Fortunately, your imagination can help predict whether or not your ideas are really dumb or really brilliant.
You're sitting down for lunch with friends on the weekend and you're feeling embarrassed about a recent choice you made that turned out very poorly. You share all the details and by the time you're done, your friends tell you, "well, yeah, that was clearly a bad decision." Everyone seems to agree that you made a bad choice now that the decision is colored in failure, and you kick yourself because you should have known better.
This scenario is avoidable with a little session of practice and pretend that you conduct with yourself. If you have an idea that you aren't sure about, imagine a conversation with your friends, family, or coworkers that takes place after you've realized the idea was 100% horrible. Think about how obviously bad it was and how you didn't see it. Do not, however, imagine why. Role playing this scenario in your head will help you draw out the possible downsides of you idea and prevent you from making what would likely be a poor choice. If you come out of this exercise feeling that the imagined failure of your idea is wrong and not a conversation likely to actually occur, you may just have a great idea on your hands.
If you're easily discourage this probably isn't a strategy you want to employ. For people who don't get too down on themselves and want to gauge whether their idea is brilliant or dumb, however, can get a clearer idea of what they're thinking by using this trick.