First and foremost hire a plumber you can trust. If you don't have a plumber to call on, ask a friend or neighbor for a referral to someone they have trusted. If you are hiring a plumbing for an installation project like a new bathroom or an addition, then its best to get some up front pricing. This will help you both budget for the upcoming costs without any surprises. Emergency and general service plumbing is usually billed by the hour.
Most companies have a first hour rate and a cheaper rate there after. (125 first hour and 75 an hour after).