Although probably not absolute one should be able to mike the barrel OD's at a given point then measure across both barrels at that same point. When this measurement across the barrels becomes greater than the sum of the two diameters then it can be reasonably safely assumed there is now a gap between the two. Barrels are normally "Jointed" in at the breeches for some distance until their taper causes them to no longer touch.
At the normal distance for a sleeved joint there would be little "If Any" between the barrels. I believe the purpose of either the weld or solder is to prevent the tubes from moving forward, not to per se add strength to the barrels. It would thus seem that on a weld job sufficient surface could be covered without having 100% of the circumference welded.
