Denny
Remember the story of Philip and the Ethiopian eunuch (Acts 8:27-40)?
Philip explained the biblical story to him and then later Philip baptizes him. In verse 36 the eunuch asked him if he could be baptized. It goes on to tell us that if the eunuch believed with his whole heart then he could be baptized.
Obviously we know that the eunuch was baptized so, we know that he was a believer, especially in light of his confession in verse 37.

In a Muslims case, I don't think it would be any different. But perhaps given that some Muslims don't believe that the Scripture understanding of Paul is Biblical. It might be best to make certain he understands that issue, clearly before being baptized.

Tom