it is absolutely possible to establish legal paternity of your unmarried partner's child, these days pretty definitively so by DNA tests.
Actually it's even easier than that.
In most states, if a person is named as the father on a child’s birth certificate, he is the legal father for all purposes. The presumption of paternity in this situation is very strong, and if the man named on the birth certificate wishes to assert his father’s rights and challenge his legal status to the child, he should do so as soon as possible.
There have been cases in which DNA testing conclusively proved that the man named as the father on a birth certificate was not, in fact, the biological father of the child, yet a court still imposed all of the attendant legal obligations, such as the duty to pay child support.
In many states, there is a short statute of limitations for challenging paternity. If the man named as father on the birth certificate wishes to challenge this legal status, he usually has 2 years or less from the birth of the child to do so.
If a person fails to challenge paternity within this period, he can lose all rights to do so at a later date. This is especially true if he acts as the child’s father in the meantime (living with the mother and child, helping to raise the child, etc.). In situations such as this, some courts have refused to even consider DNA evidence, on the grounds that a successful paternity challenge would not be in the best interests of the child.
Avoiding this situation is sometimes difficult, from an emotional and practical standpoint. If someone suspects that he is not the father of a child, the best way for him to avoid this is to refuse to put his name on the birth certificate, and to insist on a paternity test. Of course, this could strain whatever relationship exists with the mother, as it implies a suspicion that she has been unfaithful. Still, it is one of the most effective ways to avoid being the legal father of someone else’s child.
So, how do i protect myself without looking like an asshole?