We don't know, per se, in that neither has been announced yet, but theres little reason to doubt either. We have the icons for both of their series, and in Captain Falcon's case no announced rep yet, so between that and being a two-time veteran hes almost a certainty.
Marth is in a big way the face of Fire Emblem - he was its first Lord, is one of only two to star in two games (not coincidentally, Ike is the other), is still one of its most popular Lords even today (17 years after his first game and 11 after his second), and was the star of the FE animated series - so theres no better FE rep for Brawl than him (especially since hes a veteran). Theres almost no reason to doubt his return (the only one is Sakurai's mention that he'd rather not use too many Japan-only characters, which is little against him, since he was released internationally via Melee).
Zevox