Originally Posted by
Omega_DCD
I'd say the first hint was UXM 281 when Jean transferred her psyche into the White Queen's body...Xavier was surprised and intrigued, and said when he unlocked her TP, she quickly became a telepath of intricate complexity, and related that to the Phoenix Force choosing Jean
Later, Onslaught tries to coerce Jean in order to find out how her mind is able to access the Phoenix Force...she's able to resist him twice
But yeah, I agree that NXM era is when it becomes official...expanding her mind, putting Xavier's mind inside her own, losing only a few memories (granted Xavier was booby trapped with several neurological diseases...and Jean was infected with nano sentinels)...was no small feat