Proof of citizenship is....the most frequently used proof is a birth certificate...what did obama use?
I don’t know what he used. I do know that a birth certificate is NOT needed to get a passport. Null and Void has posted the link to the state site repeatedly telling of the procedure to obtain one without a birth certificate.
Also, a family passport can be used per this post:
http://web.israelinsider.com/Articles/Politics/12993.htm
In the comments by a poster named muzhik:
Obama may never have had to produce his birth certificate to get a passport. As a baby, he would have been included in his mothers passport as a family passport. Whoever took her application may not have required her babys BC its the 60s, shes white, the daddys black, and its not like the kid will ever run for president. Sloppy by todays standards, but not necessarily by the standards of the time.
After that, its just a matter of keeping the passport renewed. When he turned 14, Obama would have gotten his own passport, using the family passport as proof of citizenship. If he kept his passport renewed, he never would have needed to produce a certified birth certificate.
Still, the direct answer to your question is nobody knows. We don't know. Why do you ask?