You're checking to make sure no Polish person is standing behind you who might put a foot in your butt for belittling his brethren.
can someone explain this joke to me i don't get it Q: How does every Polish joke start? A: By looking over your shoulder
I don't get it either.
I think the Z has got it right. This seems to be a slightly longer version of the joke you listed:
A guy in a bar leans over to the guy next to him and says, "Wanna hear a Polack joke?"
The guy next to him replies, "Well, before you tell that joke, you should know something. I'm 6' tall, 200 lbs. And I'm Polish. The guy sitting next to me is 6'2" tall, weighs 225, and he's Polish. The fella next to him is 6'5" tall, weighs 250, and he's Polish. Now, you still wanna tell that joke?"
The first guy says, "Nah, I don't want to have to explain it three times."
http://www.rantnroll.com/html/polish.html
I tend to lean towards Z on his explanation but maybe....... Maybe they are "accusing/teasing" you of being polish because you will usually tend to look behind your shoulder just by hearing the joke. Maybe???