A Lv. 1 Genesect would have the moves: Quick Attack, Screech, Magnet Rise, Techno Blast but one of these won't be in its move set as obviously, only four moves are allowed, and as it is a legendary, it cannot breed and therefore doesn't receive egg moves.
We will never know which ones it knows but, if you want one that isn't in its move set you can go to the Move Rememberer and get it to remember it for whatever item is required in that specific game.