TL;DR: Press & hold to compare (iOS only for the moment).

Snapseed’s unique interface for selecting parameters (dragging up & down on screen) & adjusting them (dragging left right) long seemed to dictate that the app couldn’t use what’s become the standard gesture for toggling adjustment preview. Instead you had to press & hold the little preview icon in the upper right of the screen.

Now, though, you can simply press & hold on the app main screen (to compare the original image to the current state) or within individual filters (to compare just that filter’s adjustments to the previous state). The exception here is with brush-based filters (Healing, Brush): in those pressing on the screen obviously applies a brushstroke, so you still need to rely on the preview icon there. For its part the icon remains visible everywhere it was already.