zip memory question

hey, when i load any of the zip disc that i have it always says the discs have the exact same amount of memory? this can't be right, or is that an internal memory? if so how do i check to see how much is left on a disc, im ready to start making my own tracks on here and i wanna check to see if theres any space on the previous owners disc.
thanks

it should tell you from the load/save screen when you select the zip drive id

load / save screen , bottom right corner.