What do you do when there are no concrete examples or relevant scientific facts at the time? How do you counter a conspiracy theorist when they then also point

What do you do when there are no concrete examples or relevant scientific facts at the time? How do you counter a conspiracy theorist when they then also point out very real conspiracies that have definitely happened and are well-known and well-documented? Conspiracies definitely do happen and have been perpetrated by institutions that are supposed to be regarded with the highest trust, such as the people tasked with protecting us with their intelligence. How do you restore faith in those institutions? Would you have argued against MK-ultra before having known it was real?