Agnes Campbell - Agnes Campbell and a natural child

I have got the information from a genealogy reasercher that Agnes Campbell at the age of 18 should have got a child out of marriage with John Cunningham of West Barns. So that she in fact is the mother of John Cunningham, born 1575 in Fife, who later went to Denmark and Norway. The information about this child should have been given by her father Matthew Campbell 9th of Loudoun. Anyone know what paper we should look for to confirm this 100%? Could there be a baptizm paper, or is John Cunningham mentioned in the will or something after Matthew Campbell 9th of Loudoun?
Best regards from
Hanne W.Ringseth, Aalesund, Norway