Other contributors have said "How do you no if your dad is your dad?" is the same question as "How do you know if your dad is your dad?" If you believe that these are not asking the same thing and should be answered differently, click here
Look for similar traits, such as: hair, likes, dislikes, eye colour, etc. You could always see a doctor and ask to see your and your father's genes and ask if they are slightly similar.
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