Is it possible to falsify string theory? Luboš Motl has just listed a few ways to do so. Though I think some are too impractical to be helpful, I think it is important for string theorists to try to find legitimate was to falsify the theory so that we don't have some Russell's Teapot theory on our hands. IE... a theory that may be false but one that you can never really know.
That said, I would like to highlight one that may be semi-practical: (From Zwiebach linked to the right.)
String theorists sometimes say that string theory has already made at least one successful prediction: it predicted gravity!But actually I think the prediction string theory makes is that gravity is mediated by a spin-2 particle called the graviton. Therefore, if there are no gravitons then string theory is false.
Now, why am I claiming this may be semi-practical? Because in principle we can construct theories where gravity is meditated by something other than a spin-2 particle. One example, which I admit is probably garbage, is that gravity could be just a consequence of entropy as proposed by Eric Verlinde. And there are and will continue to be even more non-graviton theories proposed until a graviton is discovered.
Now, if one of these alternative-to-the-spin2-graviton theories are ever experimentally verified... I guess we will have done more then disprove the existence of a spin-2 graviton.... We will have also disproven string theory at the same time!
In the meantime, my money is still on gravity being mediated by a graviton as that is the most reasonable thing to believe.