Am I up voting or down voting on Hacker News

That's the upvote. Until you have (I think) over 500 karma, the downvote arrow won't appear.

I'm guessing you're new... might want to try hnnotify.com to get emails when someone replies to you.

I can confirm it's 500, i crossed that recently.

You have just upvoted.

Interestingly, I don't see the option to downvote posts, only comments (I have 758 karma).

It's an upvote.