Asking New Members about their baptism

[PresidingElder]

I wanted to ask if any one has a suggestion or two about how to effectively ask if one has been baptized according to Acts 2:38 without offending when accepting new members into the church body. I have a dear brother in Christ who has a new growing work. God through him are attracting many people from all denominal backgrounds who want to join the work and become active in ministry. Of course his #concern is trying to ask if they have been baptized according to Acts 2:38 without being offensive or challenging ones salvation. How could he ask or even verify without offending and if they have not, what verbage could be used to convey that as a requirement before serving in the ministry.