Hmmmm, read the manual... lol
if u dont have it then if there is a bracket or u know which way it would be on a wall i am thinking left->in right-> out
im no expert but if u google a litlle bit on RO units then you will know which component(filter) is first
ex. carbon or the micron filter
so once you know which is first then the input is the one closer to the first part of the filtration
but like i said before there must be something that indicates a front side(maybe a sticker) then you will know