There's a good discussion of this in Jonathan Shay's work. He has a book out called Achilles in Vietnam: Combat Trauma and the Undoing of Character. And several articles, such as "Learning about combat stress from Homer's Iliad," Journal of Traumatic Stress 4 (1991) 561-579.

I'm not an expert on the subject, but the short answer is that the evidence is there in epics and poetry. You just have to know what to look for.

Dorothy King also put together a nice discussion on her blog a few years ago. http://phdiva.blogspot.co.uk/2010/11/rage-of-achilles-and-ptsd-in-antiquity.html

EDIT: Forgot her follow-up piece http://phdiva.blogspot.com/2013/01/ptsd-in-antiquity.html

Well, PTSD is very dependent on a wide variety of factors, and simply being subject to war or some trauma is not necessarily enough to cause a case of PTSD. If it was, most of the world would have been in dire straits 1946 onwards.

As for ancient PTSD, here's a rather interesting previous response to ancient (specifically Roman) PTSD: In short, applying moder, American, standards to mental disorders is highly problematic as the same diagnoses can present different in different cultures. Classical Rome is a vastly different culture than modern America, so what constitutes trauma would be different, and how it was expressed would also differ. /u/sapere_avde would be able to give a more complete answer.