Honestly there is no proper way to check it, since SG still not properly detect many games especially dlcs as owned and thus allow entry, but if the winner doesn't have private account then you can check by his game list manually, or you can contact him by email as you can see once the winner announced. Other than that if the winner confirm the gift then there won't be problem at all to you, although idk will there be a consequence on the winner himself since it still probably violates the rule.