How to monitor google app engine from command line?
I assume you are using Linux, Ubuntu/Mint If not that would be a good start
Debug as much as you can locally using dev_appserver.py - this will display errors on start up (in the console)
Add your own debug logs when needed
Run code snippets in the interactive console - this is really useful to test snippets of code:

if you are on GAE >= 1.7.6 http://localhost:8000/console
if you are on GAE < 1.7.6 http://localhost:8080/_ah/admin/interactive/interactive
I'm starting to use Google App Engine and being a newcomer to much of the stuff going on here, I broke my webpage (all I see is "server error" in my web browser). I'd like to be able to see a console of some sort which is telling me what's going wrong (python syntax? file not found? something else?). Searching around a bit didn't lead me to a quick solution to this, so I came here. Any advice? Ideally, there would be some sort of tutorial/guide that would show how to do this.