Kennt er deinen Vater?
it might be a stupid question but how am I supposed to know this is a question? only by the question mark? am I missing here anything?
The question mark certainly helps, however you could tell by the order of the words in the sentence. Let's exclude the question mark. If it were a stating sentence, you would put the subject before the verb, as you do in English: Er kenn deinen Vater. If it were a question, you'd put the verb before the subject, you do that in English, although you use the auxiliary verb 'Does.' Kennt er deinen Vater?