On the official X-Air App when you look at the meter on the Main tab of a channel it shows the Post Fade + Mute levels for that channel only - it doesn't take into account what the fader level or mute status of the Main LR bus is. However on Mixing Station Pro it seems to also be taking the Main LR level and mute into account - so if you have the MainLR fader down or the MainLR muted the meter in the Main tab of a channel shows no level.
This means that there appears to be no way to view the Post Fade level for a channel without a signal being sent to the PA system.
Mixer Model: XR18
App Version: 0.003.0 (55)