Most trades are licensed through a state governmental agency such as a "home improvement commission". You'll need to find the agency in your state and verify that the tradesmen is, in fact, licensed and in good standing.
Do you hang the drywall, apply joint tape, drywall compound and sand?
Will you clean up and haul all debris away during and after the project?
Do you come back after the first coat of paint and point up any imperfections in the drywall finish?
Drywall nail pops occur when drywall screws or nails loosen over time. As a result the drywall moves and the nail heads push through the surface causing an unsightly blemish.