The person who told me this said that Hitler’s father was an alcoholic which made more sense, but I also have heard that the actual SS Officers were often themselves heavy drinkers or even drug users. So how was alcohol treated in 1930s/40s Germany?
Yes, alcoholics were put into concentration camps in Germany, under the category of vagrants. This was part of a steady movement by the Nazis to remove vagabonds and beggars. Your confusion seems to be in the definition of the term alcoholic. A member of society with a steady job and a family who gets very drunk at night or a heavy-drinking military officer would not fall into this "vagrant" category. In terms of this question, alcoholic is a category of homelessness/joblessness/'antisocial/unwanted element' which was not contributing to ideal society they wanted to built.
A detailed description of how this was done can be found below using Hamburg as a case study.
https://kobra.uni-kassel.de/bitstream/handle/123456789/2007022817280/vagrants.pdf?sequence=3
The answer to how did they know who were "vagrants" involves various government departments (more details in the above link) and the police working in a coordinated crackdown to eliminate unwanted member of society. There wasn't a need to prove if someone was an alcoholic or not, if they were homeless they were still a member of this vagrant category.