According to CNN:
Investigators learned Thursday [3 Sep 2015] that a series of numbers found inside the plane flaperon matches records held by a Spanish company that manufactured portions of the component, linking the debris to MH370, the office of Paris Prosecutor Francois Molins said."Consequently, it is possible today to affirm with certainty that the flaperon discovered at the Reunion Island on July 29, 2015, is that of MH370," the office said.
Since the Reunion Island flaperon, other pieces have turned up. The Wikipedia has a discussion of the marine debris.
