I think Adrien will notice Marinette drawing or writing in her Diary about Lady Bug and Cat Noir.He will get suspicious and investage and then notice that she is Lady Bug. Then hide it that he knows who she is and confess later.
I heard people are looking for a reply. I suspect that this is the way it will happen because I saw writing in her journal at the park in one episode. It gave me an idea on how it might happen. So when I found out about Lady Bug Wiki. I wanted everyone just to know how I believe it will happen.