I think the best solution would be to employ a female guard just to check who is behind the veil, even in Qatar in the past they had issues with men wearing niqab and going into girls schools/weddings and also committing crimes. Yes it will cost more to carry out such a thing and will be a nuisance for women who to choose to wear niqab but this is the only way to ensure nobody is wearing the veil as a mask to commit crimes.