When making changes to Juniper EX switches yesterday I wanted to check the changes that I had made to my configuration before committing them. A quick look in the reference manual gave me the following command:
show | compare rollback 0
This will show the edited candidate config and pipe that into the compare function and look at the changes to the specified version (rollback 0). I could look at the changes compared to a previous config by replacing 0 with another number in the rollback sequence.