First and foremost... check out qualifications. Does the coach have a license? Having or not having a coaching license does not make a coach a great soccer coach, but credentials do tell you that the coach has passed some testing and is aware of safety on the field. A license may help you make a good decision in combination with other aspects.
Experience! Ask about experience. Ask how long they have been coaching soccer and ask for references from others. An experienced coach will be able to supply that.
Safety First. Make sure your new coach knows CPR and is familiar with safety measures.
When looking for a kids soccer coach, check for fairness and equal treatment. We all know some players will get more game time, but all players, regardless of skill or experience, should get to play at some point.
Go to some of the games and practice sessions and look to make sure all kids are treated equally and given some play time.
Talk to others and parents of children using the coach, listen to their opinions as well as gripes.
Most of all, do the research and do not rush through it. A good coach may make the difference between a lifetime love of the game and a bad experience in team sports for your child.