With the trailer sitting on the ground you measure from the center of the spindle to the wheel well edge or other place on the trailer directly above the spindle. Then you jack the trailer by the frame and allow the axles to drop until they stop. Make more measurements and figure out the difference. This difference, found in Dexter articulation charts for a #10 axle is between 2 and 2½ inches, depending on the loading of the trailer (starting measurement) and down angle that the axle was manufactured with. See the second page of the link below, I think it was page 20 (excerpted from a larger manual).
If the shop did not measure the droop they have no way of knowing if something is wrong with it.
Is your trailer perfectly level when towing? Torsion axles operate independent of each other and if the trailer is tongue high when towing, this puts more weight on the rear axle, wearing those tires more, and making the trailer handle better (more tongue weight). If the trailer tows tongue low, this puts more weight on the front axle, and wears the front tires more, and makes the trailer less stable, as there is less tongue weight.
Airstreams have torsion axles and suffer the same issues.
Charles
http://www.dexterpartsonline.com/fil...%20Catalog.pdf