If I was God, we'd all be in BIG trouble.

How would I prove myself - just say hello from the sky - 'Hi guys, it's God, what's happening? No no, don't tell me, I already know, I'm God remember? I was just trying to be friendly'.
Piece of cake for God to prove himself, but he'd need to make sure everyone was paying attention, which might involve taking away free will.
No point with the sound and light show and flying donkeys and breakdancing angels and beautiful creation or small still voice if people aren't watching for it...

I seem to recall parts of the New Testament where Jesus refused to do miracles sometimes because it was just light entertainment for people and it didn't change their hearts and lives.