Benedick is the perceptive character who immediately suspects Don John as the culprit in the plot. The exact lines are as follows:

*FRIAR FRANCIS
There is some strange misprision in the princes.

BENEDICK
Two of them have the very bent of honour;
And if their wisdoms be misled in this,
The practise of it lives in John the bastard,
Whose spirits toil in frame of villanies.*

This correct assumption of Benedick's shows that he is a more rational and less emotional man than either of his friend's. He is sound in his judgement, wise, and not likely to be easily biased or overwhelmed by his situation.