Is it possible to infer the length of a neopixel string using 1-wire protocol?
Each pixel "peels off" the first data word before passing on the rest, so the payload gets shorter and shorter as you move down the chain.  Although the data wire may look continuous it isn't - the signal is electrically re-driven from each unit to the next.

For the RGB ones, they are logically three monochrome pixels of different colors, so they peel off three words rather than just one.

At any point in the chain you can determine how many logical pixels potentially remain to be updated, by counting the words you are passing on.  You can't of course tell if they are actually there, or if there are further pixels for which no data is being provided.  And you can't tell how many pixels preceded, other than to bound it by the amount of gap time from the previous update you received.