Here's a list from the creator. I'm not sure if they're all always there:
"Here is the semi-official list of symbols, more or less in order of frequency of use: eyeball, piece of pie, alien, dynamite, K2, upside-down bird, bunny head, crown, single shoe, arrow in the back.
They began in 1996 when I decided out of boredom to draw a little bird sitting on a shelf where it didn’t really belong"
Did you know that every Bizarro comic has a hidden eyeball (bottom right), alien (top right) and piece of pie (can't find it in this one)?
And always disconnect while you're in the middle of speaking.
HEAD!
You can always tell a Milford man.
He's showing off his manicure.
The date says 1988... that could be an NES Zelda reference.
You know the difference between a porcupine and a BMW?
Porcupines have the pricks on the outside.
Spot on since the publish date in the bottom corner is 2012.
Do an image search for "coughing". Ghost porn for days.
Oh! I didn't know about the number!