Okay, to see backdrop in liquify, then following the steps below would help
Select LIQUIFY from the FILTERS tab, on the properties tab, go to SHOW BACKDROP, Now set MODE to INFRONT. and make OPACITY 75%. Remember you won't see any change even after setting all these if your layers look exactly alike, without anything to differentiate then. You can work on one of the layers so you can see a difference when you select another layer.