In a forced smile, the muscles of the face tend to pull the lips mainly sidewise. A real smile pulls them sidewise AND upward, which also crinkles the eyes slightly.
Also, look at the different hemispheres of the brain. The creative side ("lying" side) controls the left side of the body, and the rational/logical ("honest") side controls the right side of the body. If it's a false smile (created), then it tends to be more pronounced on the left side of their body (your right, their left), making it a slightly crooked smile.
Don't just look at their mouth and eyes - what is the rest of their body saying? If they are leaning away with their feet planted firmly on the floor, and their arms folded as they smile in the way I just described, it's probably fake.