You would need a DNA test: that is the only way to confirm or deny parentage. If she protests or, as the custodial parent, will not allow this, you should be able to get a court order requiring the test. If the child is not yours, that may well modify or potentially eliminate your child support obligations. However, since courts do not like to reduce or eliminate child support, and there is a presumption that a child born during marriage is the child of the spouses, this is likely to be a complex case; so while you do have rights, you are advised to hire a family or matrimonial/divorce law attorney to help you.