How about saying, "You should have seen what the other guy looked like when I got through with him!"
or, "Yes, my husband does beat me regularly, but it's OK, I enjoy it, I'm a masochist!"
Seriously, though, I do think it is a bit rude to walk up to a complete and total stanger and ask them if they were abused. Common sense would tell anybody that there are literally dozens of alternative explanations for bruises, such as car accident, sports accident, toddler accidently belting you in the eye with one of their toys (this happened to me), slip and fall, and medical conditions. So why do they automatically seize on that one possible explanation? I think this is nosiness and a need for drama disguised as concern.
If family, friends, neighbours, or acquaintances ask that is a little different. They know you and their concern is probably legitimate. Doctors, nurses, teachers, and social workers HAVE to ask. It is their job and it is the law. Even they know to take the person aside privately and quietly. They do not demand to know loudly in public. Even when they are off duty they do not accost strangers on the street to ask if they are abused. They know that there are hundreds of possible reasons for bruises other than abuse.
If you are not in the mood to use one of the humourous responses and you don't feel it is anybody's business about your medical condition (as it is not), then you could try just looking the person in the eye and saying, "You are really making quite an assumption, aren't you?". Or, turning it back on them by asking them, "Do YOU have any hidden bruises under your clothing that I can't see? Are YOU being abused? Are you sure? Can I give you the name of an abuse shelter, just in case?" Maybe having the tables turned in this way will make people see how intrusive and obnoxious such questioning is.
Cog