Know different/otherwise
Definition 1 of 1
Pro Tip 1/3Conflicting Knowledge
Use when you have knowledge that challenges a commonly accepted belief or statement. 
They said victory was certain, but those who know different kept their doubts.
Pro Tip 2/3Subtle Disagreement
Suggests disagreement without a direct confrontation, showing you possess different information. 
The rumor is he resigned, but I know otherwise.
Pro Tip 3/3Evidence-Based
Implies that the speaker has evidence or experience contradicting a fact or statement. 
They claim the park is safe at night, but I know different from experience.