Other mistake: During the trial near the end, Fletcher is grilling the lover about having sex with the client every time he went over to her house. Watch his face. He starts to say something, but the scene cuts and the other lawyer says, "He's badgering the witness." It's likely the next line was the man admitting to it; however, an extra scene or two was inserted before he ends up saying it.
Continuity mistake: Audrey's cup disappears when Jerry sees Fletcher through the plane window.
Continuity mistake: Near the end, when Fletcher is giving the homeless man some change, notice how the man has a bright patch of light on him. In the next angle it's not there. Cut back to the wide shot and it's back.
Answer: As you said, he was testing to see if the wish was broken--by going up to an attractive woman and talking to her. While we don't hear what happens, he apparently said something a little too "forward" to her (probably more forward than he would have done otherwise, hence the "not as well as I had hoped"), and got slapped, so he knew that he was still under the wish's effects.
Chanteuse66
But I want to know what he said to the woman.
It's deliberately made unclear what he specifically said, because him getting slapped in the face is the gag that shows the audience that he's still under the spell. If we heard what he said, then we would know right away the new wish wouldn't have worked. It's ultimately up to the viewer's imagination to decide what he told the woman.
Phaneron ★