I don't know if I had previously registered as an organ donor previously because my drivers license doesn't indicate that, but I just registered at my state's online registry to be certain. This site helps you find your state's registry. I can't speak to the process for other states, but Arizona's took about a minute. Yay!
It's not something that is fun to think about, but you never know when you will be in an accident or something. I take comfort in knowing that my accidental death could potentially save three peoples' lives with my organs!