When you flip your hand, look at their reaction. Is their face totally dropped from a moment ago's look of confidence? When you do something unexpected to your opponent, they don't have time to collect their reactions and stuff them deep down inside. What you're seeing in that first second of reaction is a truly genuine response.
What I usually see is the face drop, followed by the posture of confidence and head-shaking; i.e. a "sympathizing" with the you that you have such a tough decision to make... Sometimes, I've seen the nervous tension met with laughter - which, while strange, is another way to break the suspense for your opponent. However, you need to pay particular attention to that initial gut reaction - it's genuine!
In my opinion, this is a very reliable tell, but you should really minimize its use to once or twice per session (or less). I've found it to help me out a TON in really tough spots!