Audio problem: When Fletcher gives Max his birthday gift, the shot changes as Fletcher starts to say, "I hope it's not partially digested." When Fletcher's face comes into that shot, you can see that his lips aren't moving.
Audio problem: When Fletcher is trying to settle the case his client yells at him saying things like "I'm the victim here". In the behind shot of her head you can see that her lips don't match what she's saying.
Audio problem: When Fletcher is on the stretcher talking to his wife and son, when the camera is behind him, his mouth is not in sync with what he's saying. (01:16:25)
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 ★