how does the narrator prove that harris enjoys the drink

From the text:

If you were to stand at night by the sea-shore with Harris, and say:

“Hark! do you not hear? Is it but the mermaids singing deep below the waving waters; or sad spirits, chanting dirges for white corpses, held by seaweed?” Harris would take you by the arm, and say:

“I know what it is, old man; you’ve got a chill. Now, you come along with me. I know a place round the corner here, where you can get a drop of the finest Scotch whisky you ever tasted—put you right in less than no time.”

Harris always does know a place round the corner where you can get something brilliant in the drinking line. I believe that if you met Harris up in Paradise (supposing such a thing likely), he would immediately greet you with:

“So glad you’ve come, old fellow; I’ve found a nice place round the corner here, where you can get some really first-class nectar.”