There are a million and one possible pick-up lines. We're partial to "Are you a parking ticket? Because you've got fine written all over you." and "Are you dehydrated? Cause you look thirsty."
But Jake apparently isn't cheesy enough to say to Evelyn "Are you a beaver? Because dam." Instead, they have this exchange:
GITTES: There's something black in the green part of your eye.
EVELYN: Oh, that. It's a…it's a flaw in the iris.
EVELYN: Yes, it's a sort of birthmark.
…and then they start making out.
But the symbolism here isn't "Gittes has no game." (He's played by a young Jack Nicholson—of course he's suave.)
Instead, the flaw in Evelyn's eye symbolizes the flaw in the system itself—Evelyn's a good character, but she's personally experienced the corruption bred by greed and by the hunger for power. Her father is stealing the city's water, and he's had a child with her through incest.
In short: from a distance, everything probably looks okay—but when you look closely, you see that something's off.
That's been Jake's experience throughout the whole movie. He sees that Hollis' death isn't a suicide and that he's been set up…after he learns that the woman who hired him wasn't really Evelyn.. These inconsistencies eventually reveal an even more corrupt and evil state of appears than Jake could've realized.
The flaw in Evelyn's iris initially suggests that there's something wrong with her—like maybe she's in on her father's scheme. But in reality, the flaw lies in the family she comes from, specifically in her monstrous father.