As more and more people join a public wave on Google Wave, adding their comments at different spots within the wave, it might become difficult to determine which comments or "blips" you have already read, and which you haven't.

Here's how.  Unread blips have a bold, vertical left border, like this:

After you click on the blip or tap your space bar, the vertical border will disappear, like this:

And yes, Wave was built so that tapping the spacebar really is the easiiest way to surf through all the blips in a wave.