Having been a victim of food poisoning a couple of times while traveling I'm a little paranoid about this.
First thing I'll do is walk into the bathroom to wash my hands. (might as well practice good hygiene) If it's filthy, I'll go somewhere else. I figure, if they can't keep a bathroom clean, the kitchen won't be either.
I did find a bathroom at a bar in Colorado that was so bad I wouldn't even wash my hands.