But it's going to hurt like hell to breath and general movements. There's not a lot they can do except for wrap you up as far as I know.
Basically you will know if it's broke. If your "wondering" then it's highly unlikely you've broken a rib.
"When I do good, I feel good; when I do bad, I feel bad. Thatís my religion." - Abraham Lincoln
Moral of the story: If your going to rob people, wear a diaper.