Children will be having a variety of non-specific therapies, and the measure needs to be able to be completed by parents, and be considered valid and reliable.
Not surprisingly, I like mine the best ;): http://www4.parinc.com/Products/Product.aspx?ProductID=PDDBI, but there are others such as the CARS. It all depends on what you want to know.
What types of outcomes are you interested in? Do the therapies focus on any domain broadly, such as language or executive functioning? For example, if you wanted to look at adaptive functioning, I would recommend the Vineland. For social communication and social behaviors, you could look at the Social Responsiveness Scale. You could also look at reduced (less in number or severity) of clinical symptoms. Hope this is helpful!
I'm assessing the effectiveness of non-specific therapies (the GAPs diet and homeopathy) therefore I'd like to find a measure of global symptomatology. Most of my research has been on ADHD where there are validated, reliable measures such as Conners' rating scales. I'm struggling find the equivalent for ASD.
I dont know if there is anything quite like the Conners, but the SRS-2 gives an overall score and 5 subscales (Social Awareness, Social Cognition, Social Communication, Social Motivation, and Restricted Interests and Repetitive Behavior). Its also well-established as reliable and valid. here is a link to the publisher's site: http://www.wpspublish.com/store/p/2994/social-responsiveness-scale-second-edition-srs-2
For such a measure, any of the above mentioned scales will do. They are all inter correlated highly. The PDDBI is both reliable and valid ..it also correlates with the Conners. It also includes a Teacher version.
Have you looked into Goal Attainment Scaling? This system allows you to develop individualized goals and measure them on an interval scale. If this process is too work intensive, I would also recommend the PDDBI (see first answer by the author of the instrument!) or SRS2. My colleagues have used them and found them somewhat sensitive to change.
As a note, the SRS stands for the social responsiveness scale (which I mentioned above). There are two versions. Like several of the responders mentioned, the SRS2 is the most up to date. You can find it here: http://www.wpspublish.com/store/p/2994/social-responsiveness-scale-second-edition-srs-2
SRS is a good starting point. It may be important to also consider cooccuring conditions/symptoms (e.g. ADHD symptoms) which may or not be the focus of therapeutic intervention.