Discuss your expectations. Musicians should dress and present themselves according to the theme of your event. Be very clear on what duties you expect of the musician, such as when to play a certain selection. Be as detailed as possible. Don't leave anything to chance.
Amplification is important.
Ask if the musicians are providing their own sound system and be sure it is electrically compatible with the venue, or is battery powered. Look for a harmonious working relationship. If the musician is not responsive to your needs during consultation, look elsewhere.