Finding the *displayed* resolution of a monitor (how many pixels are being displayed)

[99muppets]

Kind of finding it hard to explain this but, the new 480hz lg oled is available for pre-order for me at 500 AUD cheaper than the ASUS one, however, despite asking countless moron LG reps and emailing their technical department several times I still have not had a super simple question answered which is what resolution the 24 inch mode is ACTUALLY displaying. It downscales a native 1440p signal for whatever reason, and no where ANYWHERE is the actual resolution listed.

Is this even possible to figure out? by pixel counting or some other method? If i get this monitor will I be able to calculate the exact resolution / pixels that are being displayed at the 24 inch mode, is there a way to do this? Thanks