The Quran describes Muhammad as "ummi", which is traditionally interpreted as "illiterate," but the meaning is rather more complex. Medieval commentators such as Al-Tabari maintained that the term induced two meanings: first, the inability to read or write in general; second, the inexperience or ignorance of the previous books or scriptures (but they gave priority to the first meaning). Muhammad's illiteracy was taken as a sign of the genuineness of his prophethood. For example, according to Fakhr al-Din al-Razi, if Muhammad had mastered writing and reading he possibly would have been suspected of having studied the books of the ancestors. Some scholars such as Watt prefer the second meaning of "ummi" - they take it to indicate unfamiliarity with earlier sacred texts.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What skills would have made others more suspicious that Mohammad was not getting his revelations in the way he said?
writing and reading