A method for a rapid verification of satisfactory clean-up at the end of the completion process when starting production of petroleum fluids while the completion rig is still at the well. The method includes marking one or more of separated zones along the well by one or more distinct tracers, before starting production. The tracers are added along with the completion or the clean-up fluid, and the tracers are arranged to be released by relative high concentrations from each zone to an inflowing fluid of oil or gas.