Sector, I asked the same question when I had blood drawn for testing, and was told that I have to tell my doctor and request a test be done to find out. I couldn't have it done on that occasion, because the blood that was drawn was more like a diagnostic sort of thing. (Which, I might add, turned out to be something stupid that doesn't show up on a blood test anyway.)
I don't know what type I am and have yet to donate blood. I just have had it drawn a couple of times for testing. It's a little bit awkward when a person who works there begins to recognize you... >_<