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?
You can tell by the word order: when the verb comes first, it is a question! Compare: a) Er hat einen Hund b)Hat er einen Hund?
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?