Hi Kate,
There are different kinds or versions of TOS.
A basic description is something causing impingements or compressions on the nerves or blood flow in the brachial plexus areas{ scalenes, clavicle, top rib, extra cervical rib, large muscles or ligaments, etc}

Here's some of my fav websites that helped me to learn, besides the forum.
TOS info:
http://www.medifocushealth.com/RT017/index.php
http://www.nismat.org/ptcor/thoracic_outlet/
http://tos-syndrome.com/newpage12.htm
http://tos-syndrome.com
http://www.doctorellis.com
http://www.cwce.com/feinbergarticles/tos.htm
http://www.tellmeabouttos.com
http://www.vascularinstitute.com/Resource_TOS.htm
http://drbrantigan.com/about/index.htm
http://www.ecentral.com/members/rsanders/
http://www.causeof.org/posture_tos.htm
http://intraspec.ca/tos.php

There isn't a clear perfect test for TOS, unless you do have the extra c rib or bony structures.
It is usually a process of elimination of other causes - like cervical spine injury and such, then add up all symptoms.
But very much like RSD you need to have knowledgeable people working with you. And if you happen to have the combo of TOS- RSD- Fibro you really need some experts or ones that will want to learn and treat you one on one, not with the standard treatments that work for acute injuries.