A clean compliment can make her sound too available too soon. A tease gives her room to move.

She says his shirt is trying hard. She asks if he always looks at people like he is solving a case. She picks something he can answer, not something that shuts him down. The point is not to insult him. It is to see whether he can take the hit and send something back without getting awkward.

You can hear the difference after his reply. A flat jab dies there. A flirty one leaves her smiling into the next sentence.