In the previous world wars there were various alliances between many countries, but eventually two sides emerged. If you see two clear sides engaging in active warfare against each other, you’ll know that WW3 has started. At the moment, it’s a bit more complicated than that. Seems to me that there are a bunch of disconnected conflicts going on, but the big picture of a proper world war hasn’t really emerged yet.
I’m a very picky hippie when it comes to dimensions.