It is a little hard to say, since I have never read this thinking it was serious.  But I would say that you really start to suspect in the paragraph when Swift starts talking about selling the children.  He talks about how much they could sell for and that just doesn't seem realistic at all.

However, you can't really be sure (he could have been talking about selling them as apprentices or indentured servants) until he starts talking about how good they taste.  At that point, it's pretty obvious.

His real proposals come towards the end of the essay. They are the things that he rejects as crazy.  This is the part that starts "therefore let no man talk to me of other expedients:"