Very true, you may smile but that's not what you feel on the inside but I think there is something that always gives it away (That you aren't truly happy) because its hard to keep up the acting when you aren't feeling the way you act.
The work place is the best example of this.
I could walk through our office and exchange a dozen smiles with people, each of whom I know to a different degree.
Aside from being a pleasant way to present yourself the smile doesn’t reflect much about what is going on inside.
Krusten’s right that there will alsways be something that gives it away but in some cases you might need to have an in-depth conversation with someone you know we’ll to figure out what it is.