Just open a Terminal Emulator window and run it full screen. However what you probably need to see is the errors that get generated on the console you launched startx from.
In that case you can see the error or warning log file for troubleshooting purpose at the following location:
In your full screen Terminal window use the tail command to view log file:
tail -f /var/log/Xorg.0.log
That said, I am not sure if the Xorg log file will give you everything you need. (In fact after looking at it, it appears to only have start-up messages) You may still need to flip to the console (Ctrl+Alt+F<x>) you launched startx from to see everything.
I would think there would be a way to see the information printing to the console you started X from in a Terminal window, but I could not find one. I did find a verbose option that may give you more info in the log file: