Connect several clues to an interpretation, then check whether another explanation remains possible. An inference can be well supported without giving access to a character's unspoken thoughts.
Worked example
A speaker rehearsing twice and checking notes may be preparing carefully. That does not prove the speaker has never presented before.
Original fiction: Before the Demonstration
Mira arrived early for the demonstration. She arranged her materials in the order she would use them and practiced the opening twice. When the room filled, she glanced at a small note card before speaking. Her explanation was steady. The narrator does not tell us whether she felt nervous or how many demonstrations she had given before.