It is very hard to fool Chris Mannix
He deduced the Lincoln letter was a lie. It was a lie that fooled John Ruth. The Lincoln letter was very well-written and looked authentic.
He also saw through Daisy and her remaining men's lie that there were "15 men waiting in Red Rock" if this attempt to free her was unsuccessful. He saw it was a bluff and called her on it when she was the last one left.
It is very hard to fool Chris Mannix.