Stay with the selected viewpoint. The character can know their own thoughts and notice another person's words or actions. Treat guesses about another person's thoughts as guesses unless that person reveals them.
Worked example
From Nia's viewpoint, 'I saw Ben hide the envelope' is an observation. 'Ben hoped I would never find it' claims access to Ben's private thought unless he tells her.
Nia Notices an Envelope
Plan a first-person scene told by Nia. She sees Ben glance toward the doorway and slide a yellow envelope into his bag. Nia has not seen its contents. Ben has not explained why he is hiding it. Choose sentences and plot moves that respect what Nia knows.