Hi weiqun ding
You can find the definition for the idiom smoking gun on this site: "definitive proof of someone's guilt"
You can find the definition for tableware in any dictionary. Tableware consists of the dishes, glasses, utensils, etc. used at the table.
So, in your article, the water glass and utensils that the suspect used at the restaurant provided indisputable proof of the crime.
Student or Learner