These six verbs can describe becoming aware of something, but they emphasize different parts of the experience. Their meanings overlap, so more than one may fit a situation.
- Notice means that something comes to your attention, often through sight or hearing. I noticed a crack in the window. The focus is on becoming aware, not necessarily understanding why it matters.
- Realize means to become aware of a fact or understand a situation. She realized that she had left her wallet at home. The focus is on understanding, rather than simply seeing or hearing.
- Recognize often means identifying someone or something from previous experience. I recognized his voice immediately. It can also mean accepting that something is true or important: They recognize the need for better training.
- Perceive can mean becoming aware through the senses, usually in more formal language. He perceived a faint movement nearby. It also describes interpretation: Some customers perceive the new packaging as wasteful.
- Discern means managing to see or understand something that is not obvious. It is fairly formal. I could barely discern the path in the fog.
- Detect means finding or discovering something, especially something hidden or difficult to notice. The test detected an infection. People, animals, and devices can all do this.
Compare I noticed her expression with I realized she was upset: the first highlights attention; the second highlights understanding. Choose according to what you want to emphasize, not a rigid rule about the situation.