Dear LIB readers; if you suspect a baby isn't yours, how do you go about it?

From a male LIB reader.

My girlfriend gave birth on July 3rd but I am convinced I am not the father of the child. For one, the boy looks nothing like me, and secondly, I didn't sleep with her nine months before the baby was born. We had sex in early September last year before I left for Germany and I didn't see or touch her till Christmas 2014. She gave birth July 3rd, ten months after we had sex. Isn't a woman's term of pregnancy just nine months? If she had given birth in June, I wouldn't have bothered but as it is, I have my suspicions but don't know how to say it to anyone in case I am wrong. What do I do?