Another good way of seeing if the saddle is a good fit is to ride in the saddle using a saddle blanket/pad underneath. When you take the saddle off, look at the saddle pad. There will be an imprint of the saddle from the horse's sweat on the pad. The damp patch should be uniform; if there is an area that is damper than elsewhere, or drier than elsewhere, the saddle does not fit. Also, consider a saddle with short rather than long girth points. Long girth points can add extra stress/strain to the horse's back, ribs and girth area, and you will find that horses that are ridden in saddles with long girth points tend to suffer more girth problems than horses ridden in saddles with short girth points. This is an observation though, not a hard and fast rule.
Lovely that your trainer has done a seminar/demonstration on saddle fitting, but then so have I, and I certainly wouldn't call myself an expert. The only true saddle fitting experts are the guys who make the saddles. Would you trust your car's mechanics to someone who has 'attended a seminar', or someone who has served his/her apprenticeship and is fully qualified?