This achievement is unlocked on your first case after earning traffic detective.
When you arrive at the crime scene with the suspicious car, there are multiple clues that you can inspect. A few of the clues include opening the trunk of the car and picking up the pig receipt, picking up the lead pipe near the passenger side of the car and observing its symbol, and looking inside the car and its messy interior. Looking at either 3 of these clues will unlock the achievement.