A Dash 8 Captain reported being refueled over-wing and he noticed the fuelers taking a long time to accomplish the task. After takeoff the fuel cap lifted out and was on top of the wing. Flight returned to departure airport uneventfully.

Narrative

We notified the station over-wing fueling was needed. Fueler did not show up for 45 minutes; that made it departure time. The fueler took fifteen minutes to figure out how to use the truck they brought. I noticed the fueler took a long time replacing the fuel cap on the left wing. At this point I should have had maintenance come and check the security of the gas caps; but because we were already thirty minutes late I decided that the fueling professional got it right. After liftoff I noticed the fuel cap had lifted out and was on top of the wing. At this point we were still with the tower and explained our situation and that we would like to stay with him and return for landing. We were turned down wind and cleared to land. The Flight Attendants and Passengers were notified and all checklists were completed. I think the fueler was not trained for all of the special needs of our aircraft and a supervisor should have done the job while instructing the fueler. I should have had maintenance inspect the fuel caps before departing. After the fact our maintenance personnel checked the other wings cap and he found it was not installed properly. Train all fueling supervisors and require them to do all over-wing fueling or have maintenance required to inspect after over-wing fueling is done.