Aside from someone else clueing you in, wouldn’t you notice after a while that you don’t get any upvotes anymore? (or downvotes for that matter)

This wasn't clear?:

"A few people messaged me on other channels to let me know."

How did those people know?

You can set "showdead" to "yes" in your profile. (This option might only be available beyond some karma threshold.)

It's available to new accounts.

In your profile you can set an option to "show dead" - this shows hellbanned users.

I just turned this on. What differentiates these comments from others? Do usernames show up in a different color or something like that?

It was quite some time ago that I was hell-banned, but I think it was because my account got progressively slower, and I wasn't getting any replies or upvotes. I think I might of googled 'why is hacker news slow', and that's how I found out.

are you sure? see my other reply to you here. you can easily check if you are hellbanned by logging out and seeing if you can see your posts. the site is sometimes slow and often people don't reply...

Ah dang. I don't think I was actually. I don't think I'll go back to my old account though - I have karma on this one that I'd prefer to hang on to (in case I make a stupid comment one day and get down-voted heavily).

This was a while ago, but I think I had read about ghost banning on HN before it happened. I'm assuming the same for the people who messaged me.