You're asking that person how they can be sure if Adnan knows whether or not he strangled Hae Min Lee to death?
Jay and Jenn know if they are lying.
I know the police are so corrupt they might have framed an innocent child just to close the murder case of a teenage girl so they could get back to their donuts and racist jokes, but no one is alleging (though maybe someone has, honestly) that the police are so amazing at their conspiracy, so adept at corruption, they convinced Jay that he actually helped bury Hae Min Lee in a shallow grave when he was just sitting alone in his room contemplating his next pet purchase.
Adnan knows if he killed Hae.
Jay knows if he helped bury Hae.
Jenn knows if Jay told her about the murder when she picked him up from Adnan and if she drove him back to wipe fingerprints off the digging tools.
If you go that route, read Jenn's transcript from her police interview, where she implicates Jay in the burial before Jay had talked to the police. This is a distinct moment that shows Sarah Koenig mislead her audience.