it's not something that we notice right away, or at least I don't notice right away. I have to remember to check if there is a ring. And yes, most of us know what an engagement/wedding ring looks like. We even know that it varies between western and eastern cultures! (waw! we kow so much...lol)
but I also agree that people may mistake a regular ring for a wedding ring, so better not to wear one if u don't want that to happen. You are also right in that we sometimes ask, or check FB (lol), but that's not usually the case.