The menu bar is on the 27 inch IMac. If the secondary Browser monitor is turned off nothing happens when I hit command/F12. When I reconnect that monitor the full screen version is sitting there on it. Seems weird that when you've set up dual monitors there is no way to set which monitor will be the full screen one.
Command F12 is simply the command to "Play All Frames"
The place where you select the output path for FCP is in: View>Video Playback.
There will be a list of options there. (The list can be quite long if you have an 3rd party card/breakout box installed.)
If you want the material to fill your main monitor, the option you would select from the list is: Digital Cinema Desktop Preview - Main