Look for what's not being said directly but suggested or hinted at when identifying subtext. She said she was fine, but the subtext was her disappointment.
Pro Tip 2/3
Reading Between Lines
When you read or hear something, consider the context and background to understand the subtext. His compliment had a subtext of sarcasm given their history.
Pro Tip 3/3
Not Just Words
Subtext can often be found in a character's actions, silences, or decisions, not just in dialogue. His abrupt departure added a subtext of anger to the scene.