Had the person who died in Liege in 1783 lived there all their life? If so, and you have the address, then the Catholic church should be able to find a parish register that records the person's baptism, marriage and date of death, unless the parish concerned lost their records during the French Revolution.
Turin is more complex - but Sardinia-Piedmont or Piedmont-Sardinia was likewise a Catholic kingdom at that time so the marriage of two Protestants must either have taken place in a Protestant chuch if any existed there at that time, or in a consulate or embassy of a Protestant kingdom in Turin, as I doubt that civil marriage existed there then.
If you know the nationality of the Protestant couple either a historian working mainly with diplomacy of the time or a historian of religion specialising in Protestant minoirities in the area might just be able to help you. If you know the birthplace of one of the two who married in Turin the church where either was baptised may have an annotation in the parish register of the person's subsequent marriage.