The complete question is as follows:
An edge of a flow network is called critical if decreasing the capacity of this edge results in a decrease in the maximum flow. Give an efficient algorithm that finds a critical edge in a network.
I believe some variation of Ford-Fulkerson would have to be used over here, however I am not too sure. Also I am a little confused by the wording of the question. What does efficient mean? In linear time i.e. $O(|V| + |E|)$?