To find out if their father is Brazillian, you could always ask them. If they are unsure, they can ask their father. Another way to find out is to look at the birth certificate. It should list the country of birth.
Like A Birth Certificate.
Here in the Philippines, a mother can not just name any person as the father of her child in the Birth Certificate. They will look for a marriage contract as their reference when you named your child under his/her father's surname. Or if not married, they let the father fill up the Affidavit of Acknowledgement/Admission of Paternity at the back of the Birth Certificate.
Having one's signature on the birth certificate is a good first step, but it will not automatically give custody to that man. The court will look at all of the circumstances of the case.
Actually, the birth certificate says "African" -- and it refers to his father, who is in fact from Africa (born and raised in Kenya). The confusion is that the president has the same name as his biological father. The "African" refers to his dad, Barack Senior. The term "African-American" was not in common use in 1961, so it would not have appeared on a birth certificate. President Obama was biracial (mother was white and from the United States, father was black and from Africa). But his birth certification does not classify him-- it classifies the races of his parents.
Look at birth certificate
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Airplane.
Look at your birth certificate
Go to the County where you was born, to the Hall of records. Ask for a new birth certificate. Call them first and they will tell you what you need to bring with you, in order to get your birth records.
You'll have to have more than a Christian name of your father and need the Surname (last name.) Try contacting Vital Statistics where you can look up your birth records and see who the father is on the birth certificate.
The only way to tell a person's age is to ask them. You can get a rough estimate by looking at them.