This is something that you should risk assess as part of your pre-requisites. It's all about risk assessments.
As Ian R says, it is part of an emotive subject; where do you stop with regards to religious ornamentation? I looked into this when I developed our pre-requisite program, and to be honest, it's a mine field; your either being racist or compromising food safety!
You can treat it as you would any piercings or jewellery. Alternatively, you can stipulate that they use a safety pin, count in and out of shift and make sure it's detected by your metal detector (make sure you document this) .
Interestingly, this is what Tesco say in their TFMS V5 (8.9.1)
Where hair covering is worn for religious reasons a risk assessment must be completed and controls in place to manage the risk of product contamination