Is there a sin upon a young man who works in hotels if guests who commit the abomination of adultery stay with him without his knowledge, and what should he do about his earnings from this work?

You are obligated to check the family card. If it becomes clear that the woman is not the man's wife, then it is not permissible to accommodate them. As for those who act deceitfully, or whose true situation is not known until after their departure, their sin is upon themselves, not upon the workers. There is no sin upon you for working in the hotel, and what you earn from it is lawful.