I watched my kid get born, and kept an eye on him as long as I could. I identified whatever birthmarks and characteristics I could before I let them out of my sight. My wife insisted :)
Anyways, its not too hard to mistake them as mine, both boys have a certain "look" to them. The nurses all commented about it. (fairly prominent eyebrow ridge, along with a "what you lookin at, punk?" look....)
Isn't that funny that you can see that stuff in your kids even when they are newborn? I could make out both my boys personalities almost from the day they were born.