How can you be sure that an ad hominem attack or other inappropriate invocation of the Nazis doesn't take place in a history subreddit that is also chock full of legitimate references?

Godwin's law doesn't talk about only ad hominem attacks though. It talks about comparing someone/something to nazis, most often it'll be related to the content the comments are on. Most of the content outside r/history is unlikely to be originally related to WWII. I would count a random mention of nazis under a trump article as Godwin's law.

Although, knowing reddit most of the comments are probably along the lines of "I did nazi that coming".

Source: I spend a lot of time on reddit