Remember when Sasuke blew up Deidara? Tobi was really close to the explosion,pretty sure he was in it too. Tobi depended on Deidara's guidance,so Tobi couldn't have fended for himself. If Tobi was Obito,Itachi would've known right off the back. Tobi died along with Deidara,Obito saw this as an opportunity,and took Tobi's place. If you don't remember,Tobi never returned back to the Akatsuki hideout,why you ask? Because Obito didn't know it's location. If Tobi survived he would have returned and told everyone about Sasuke killing Deidara. And Obito found Sasuke pretty easily,especially after he used Kirin. So,yea,Tobi isn't Obito.