Thanks for all the responses!
How do I know that I don't have any milk? Well, I guess I don't know for sure, but my DS tells me that I don't (he actually said he wanted me to go to the store and get some milk and put it in my "nursies") and I can't hand express anything - not even the tiny drop that I used to be able to. I also had many supply problems with him in the first year, although I was fortunate that I never had to supplement.
Why do I continue to nurse? Because my son wants to, and I don't have a reason not to.
Thanks again everyone for the responses! I guess I am mainly a little concerned about nursing once I get pregnant (my DH and I started trying for #2). I was soooo sick during my last pregnancy and for about the first 10 months after my son was born, everytime I would nurse I would get nauseous. I'm worried that his nursing would trigger morning sickness during pregnancy and I want to think things out in case I have to wean him.