Ok, Saitama found out King is powerless. Do other people know about it? How about the other heroes he hangs out with? (Genos, Fubuki, etc.)
It seems only Saitama does. We don't know about other heroes Genos envisioned all the heroes including King fighting together so it seems that he doesn't think of King as a weakling. King asked to train under Bang and it does seem that Bang had got suspicious of King but it still seems like nobody knows of King's weakness.
From now, as we saw in the last episode, we can suppose that Saitama, Genos, Bang, Bomb know about King's true power. They saw the entire scene, with King shouting "Saitama-shiiii" and asking for Saitama to help.
Reference : Episode 12 Season 2