You need to find yourself a friend that will be brutally honest on just how bad or offensive your jokes are. I found someone like that quite accidentally and it's really improved the quality of my humor from other people's perspective.

I'm still trying to figure out what makes some stuff funny and others not. For example randomly yelling "You're fuzzy!" at a cat during a heated discussion works if I use it sparingly around my close friends. Other people just give you weird looks. (My rule of thumb: most humor is best used sparingly and definitely around the proper audience.)