User blog comment:The Koromo/Chapter 758 Review: Again/@comment-1595565-20140828180901/@comment-122.105.120.10-20140829230604

Nobody700 DP was right about Usopp not knowing haki, in this chapter he was surprised to see the aura's off luffy, law and sugar. This would suggest that this is the first time it has happened for Usopp.