Corrected entry: Shrek mentions at the wedding that Farquadd is just marrying Fiona so that he can become king. However, Shrek was never informed of this during the movie, nor would he have had any chance, so it is impossible for him to have known this.
Phixius
4th Apr 2010
Correction: He knows this because he's not completely ingnorant of the politics of his world. He knows how things work there. Fiona's a princess; Shrek knows this. Farquaad is ambitious and wants to marry Fiona, whom he's never met; Shrek knows this. He simply put two and two together.
Phixius ★