The measure of outcome is always subject to debate. There are many questionnaires that were developed to measure the outcome of a therapy and they can be on many variables. I think that one of the usual measure of outcome is to see if the numbers of symptoms that fit the diagnosis for PTSD went down, if distress or anxiety went down and so on.
There's a website on EMDR and there's a part on efficacy :
http://www.emdr.com/efficacy.htm