I suspected him a lot from Chapter 4:

"In one respect, perhaps, Mr. Elton's manners are superior to Mr. Knightley's or Mr. Weston's. They have more gentleness. They might be more safely held up as a pattern. There is an openness, a quickness, almost a bluntness in Mr. Weston, which every body likes in him because there is so much good humour with it -- but that would not do to be copied. Neither would Mr. Knightley's downright, decided, commanding sort of manner -- though it suits him very well; his figure and look, and situation in life seem to allow it; but if any young man were to set about copying him, he would not be sufferable. On the contrary, I think a young man might be very safely recommended to take Mr. Elton as a model. Mr. Elton is good humoured, cheerful, obliging, and gentle. He seems to me, to be grown particularly gentle of late. I do not know whether he has any design of ingratiating himself with either of us, Harriet, by additional softness, but it strikes me that his manners are softer than they used to be. If he means anything, it must be to please you.