"...asparagus makes my urine smell funny." -Babe Ruth
The chemicals which make urine smell bad after eating asparagus are S-methyl thioacrylate and S-methyl 3-(methylthio)thiopropionate. Those people who produce these compounds after eating asparagus have a gene which allows these chemicals to be produced while the asparagus is being digested.
Roughly 40% of the population has the gene, and 6% overall have it twice. Those that have the gene twice seem to produce more pungent urine than those who only have it once.
An experiment at the Hebrew University of Jerusalem where people were asked to smell different dilutions of "asparagus urine" found that most people could smell it if the dilution was stronger than 1 in 16. About 10% of people could detect it at much lower concentrations. Since most of us pee into toilet bowls filled with water, you'd have to have a pretty good sniffer to detect any strange whiff coming out of the bowl.
So, if eating asparagus makes your urine really reek, you probably either have two genes to make the scent or are particularly sensitive to the odour. So you can brag that you and the great Babe Ruth have a common genetic heritage.
adapted from The Science of Everyday Life by Jay Ingram