[justlurking]

Someone on the plane probably knew death was coming and wanted everyone to see what happened..but how did investigators know which cell phone to check?

Short answer: They check all of them.

Long answer: The investigators would collect every cell phone they could find, and examine every one that is still intact. They are evidence of a crime, that could be used to piece together what happened.

If they hadn't recovered the cockpit voice recorder, it might have been the only evidence.

[Riflema]

Exactly, it’s been SOP to develop all rolls of film found in airliner plane wrecks for decades - bringing it up to date, why not look in every cell phone?

[NEMDF]

Yes, I agree and remember thinking that several days ago. That they would collect and check ALL of the cell phones.

[Slambat]

“The investigators would collect every cell phone they could find, and examine every one that is still intact. They are evidence of a crime, that could be used to piece together what happened.”

....that could be used to piece together or
cover up what happened.