Unfortunately, DevForce does not handle many-to-many relations well. Each entity does keep an internal ManyToManyChangeMap which contains information on all m-m relations, but this map is not publicly available, and even if it were, quite abstruse in how it's used. There's probably not a reliable way to detect these changes on your server, but if you do see unchanged entities in the save interceptor prior to execution of the save, they will be part of an m-m relation.
For m-m relations only, doing a clear on the navigation property, eg, A.Bs.Clear(), prior to the load or re-query should work.