you have a dedicated button on the surface for Pre/Post, pressing this button views the pre post setting for selected mix,
pressing Pre/Post and Select changes pre post for the active mix
the routing screen uses different colours for pre or post
Pressing the green “Pre” button on the surface shows you the current state of every channel dependent on the mix you chose (f.e. Aux 1).
Holding Pre and pressing Sel on a channel changes Pre/Post on the channel, holding Pre and pressing Sel on a Mix-Master chancels Pre/Post for the complete Mix (Aux).
It’s simple and fast and it’s there since early iLive-days (also on GLD and dLive).