My substrate "bubbles" occasionally as trapped gas is released, but usually it's in pretty big bubbles.
Plants that are "pearling" will usually show little beads of oxygen on the leaves/stems. There is also "streaming" which usually is oxygen being released from some damaged tissue in the stem.
I'm still learning about this stuff, so someone else may chime in with more info.
Here's a video I took last year with one of my plants (Hygro) releasing a steady stream of bubbles: http://www.youtube.com/watch?v=E80C-iwaaMM