A bug exists only if ALL of the following three conditions are met:
1. We know the expected result
2. We know the actual result
3. We know that the actual result deviates from the expected result
Every time you see a mismatch between the actual and expected, label that situation with the word "bug." As time goes on, you'll develop a habit, which will gradually grow into a reflex. Please note that for the sake of this training, it doesn't matter how futile, cheap, or temporary your expectations are - the main thing here is to gain the skill of automatic bug recognition.