Here are some useful commands you can put in the console. The console is opened by pressing the tilde (~) button on a standard English keyboard, but it could be different if you use another language standard. It tends to be the button below Esc however.
If your marble reflects the sky:
To check what your FPS is:
To play a .rec:
Note: Demos play automatically if you leave the start menu up for 10-15 seconds; they are played in reverse alphabetical order. This doesn't seem to work properly on all versions of MBG however so you may have to use one of the commands.
To check the elapsed time of a .rec:
You can also check the final time after the .rec has finished and sent you back to the level select menu.