List what the narrator directly observes, then separate interpretations of those observations. Consider an alternative explanation supported by the same passage. A narrator's confidence is not itself evidence.
Worked example
'They stopped talking when I entered, so they dislike me' combines an observation with an assumption. Preparing a surprise could explain the silence too, if other clues support it.
The Covered Poster
When I entered the club room, Jo covered a poster with a jacket. Clearly the club had decided to replace my design. I left before anyone could explain. Later, Jo sent a message asking how to spell my surname for the design credit. I still felt sure the jacket meant rejection, although I had never seen the poster beneath it.