As Richard and dhasenan said above, there was rather small number of suspects:
- Only small amount of population was wizards,
- There was even less people, who openly and actively fought Voldemort, especially young women that were to give birth in such narrow time interval.
But there is something more, that many people don't consider or remember. Part of prophecy says that "Dark Lord will mark him as his equal", which in my opinion have big meaning. Even if there were more children, the Dark Lord kind of predestined the future by choosing Harry.
The thing is, we can never be sure if there couldn't be anyone but Harry. Prophecy don't precise it, so it could be that Harry dies permanently, then is some time appears another powerful wizard, born at the end of 7-th month in that time, whose parents was opposing Voldemort, and who after appearing reveals his past. Dark Lord could start to be afraid of him and starts thinking of him as a danger, "mark" him in some fight or event, which can be treated as acknowledging him as equal.
It also still could be Neville, but I think that after "marking" Harry Voldemort never thought of him as equal nor was afraid of him. There could something happen and change this matter, but I cannot think of anyone, maybe except Dumbledore for time being, that Voldemort could be really afraid of.
Voldemort deducted that it probably was Harry, maybe he would even try to kill Neville afterwards. But because of that fear, that someone can abolish him, endanger his "immortal" existence, kill him, he tried to kill that person in order to secure himself and get rid of possible and grave threat to his life. Because of that he marked Harry as equal, which was even mounting up for all these years.
So for me, Voldemort's choice, perception and further life significantly prepossessed on flow of things and mostly predestined the ending result. He chosen Harry to be equal to him and bound himself to him, only strengthening this "link".