On the Sandbox: Simply use the Debug-menu on the top-right. Open the wave on Google Wave and click on Debug -> Get Current Wave
ID. Make sure you use the full wave id (something like
wavesandbox.com!w+MNJWKLNa%A) (Thanks to
Michael Kamleitner )
On the wave preview (Or when you don't have the Debug menu): Open the wave and look at the location bar. You will see something like
this:
https://wave.google.com/wave/?pli=1#restored:wave:googlewave.com!w%252Big2e7T0UC.13
The wave id is the part after wave:, in this case, googlewave.com!w%252Big2e7T0UC.13 You will have to take the part after the '.' out, and replace %252B with
+ so in our case the final wave id would be:
googlewave.com!w+ig2e7T0UC