You know you are a nerd when someone in your life asks you: “another new X, don’t you already have a 100 of those” on a regular basis.
You can perfectly be a nerd without people asking it. You can also be a nerd without broadcasting with a flashing sign, here look a nerd. It might come out if you talk with someone, but you don't need to bring it up everywhere you go.
Moderation and being a nerd are not opposites. Like everything, there are people who love to amass material possessions (in such amount that to any external eye might seem like a pile of worthless stuff) and there are others that try to keep to the essential (that others might even question if they really love technology so much since they haven't bought a new gadget in yea. Neither should be judged if it's not brought to its extreme. That also doesn't mean that one of them is a nerd and the other is not.
I am a nerd, but I'm also a minimalist. I don't often buy the latest gadgets, I prefer to stick with a few tools that I use to their fullest and know that work rather than having 100 tools just barely touched.
It's more of a mindset problem that just being or not being a nerd.
PS: Never liked the term nerd regardless.