Direct broadcast, Nintendo announced series designer Masahiro Sakurai is to debut screens of the new entries at the LA expo in June. Sakurai then clarified the news on Twitter, hinting there'll be more of the inter-icon fracases than just screens.
Sakurai tweeted he'd like to show both the Wii U and 3DS versions, and that, while "other games" have had visuals shown, he'd like to "release something better." It would be a bit surprising to only see screens, given Melee and Brawl both were first showcased with trailers at E3 expos.
Nintendo also announced new Wii U Mario Kart and 3D Mario games are in development, and are to also be shown off at E3 2013. In their case there'll also be playable demos at the expo, but at this stage that seems a pipe dream for the new SSB games.