I'm sure you've heard of at least one of those blatantly wrong sayings like you can tell if a smile is real based on if you can see eye wrinkles and so on and so forth. The problem is, if you seriously need these kinds of tricks to check if a smile is real, you must be really dense. There is a huge difference in attitude if a person is genuinely happy, being sarcastic, or just humoring you.
If you've heard of those sayings, the person you're trying to check has most likely heard it too and if that person is seriously trying to trick you, you'll be played like a fool.
And what is exactly is the point in knowing if a smile is really real. Happiness is contagious and if you start smiling, a fake smile might turn into a real one. And that's also true visa versa. If you've practiced your fake smile to the point that it appears real, you might even begin to really feel happy.