There is functionality in consensus/replay_file.go for stepping through a consensus log one message at a time in a console to get a feel for what happened. You can also just playback the whole log.

The console is really good for understanding how the consensus implementation actually works. But the code is all untested (it uses slightly different code paths than the main routines for playback). The functionality is exposed in the command line, but currently disabled. We should write tests, documentation, and a blog post about it (blog post can show how to use replay console to step through a consensus log, would probably be very illuminating). But first we need the tests.