Do you mean legal to work in Hong Kong without getting permission from Immigration? Ask to see their permanent HK ID card or a dependent's visa. If they can only show you a domestic helper visa or some other work permit, then they are not free to work for whomever they wish in Hong Kong. For example, if you are hiring a nanny, some of the applicant's have a work permit to teach for a certain school, this does not entitle them to work for you as well.