They know because of the algorithm:

Hear a thing and store it and the associated vibe - yay/nay.

Step 2:

Mindlessly repeat stored information and vibe when it feels appropriate.

Step 3:

Wait for somebody else to do the work of refuting/verifying your info + vibe.

Step 4:

Go to step 1.

When you realize this is what all people are doing almost all of the time (and many, all of the time), you are liberated.