There is usually a document called a service level agreement in place which is an appendix to the contract between the airline and the fuel supplier, this determines the level of activity that the refuelling operator can carry out with and without supervision. The different service levels reflect the level of airline and aircraft specific training the fuelling operator has received.
A good starting point to look for a description of these service levels is the manual JIG 1 (Into-plane fuelling services) and refer to Appendix A11 IATA Levels of service. Aside from this agreement you would need to be aware of the risks around fuelling (e.g. spills) without airline staff in attendance (dare I say carryout a risk assessment) and how assistance would be called to deal with such situations. Also consider is this activity permitted by the airline and by the airport authority?
Since JIG is not the only operations standard used around the world then it may or may not apply at the OP's particular location; however I would expect that other Op Standards will cover the situation in a similar way.