Trial shifts are common in hospitality as part of the recruitment process. Some trials will attract payment of national minimum wage and some won't; it depends on what is done during the trial shift. If you ask a person to do tasks, as part of the trial, that provide benefit to your business, then the person undertaking the trial will be entitled to receive national minimum wage for the hours spent because what they are doing is classed as work. 

If what they are doing is not considered to be for your benefit, but a pure test, then there will be no legal obligation to pay them the minimum wage. For example, if you ask someone to wait on tables, or prepare food that will be served to customers, that will be considered as work and therefore attract minimum wage payment. On the other hand, if you ask a chef to come in for a trial and, as part of it, ask them to prepare a three course meal that will be tasted by you and not served to customers, that will not be classed as work and you will not be required to pay them.