What does a good weld look like
I found This site about welding to be somewhat helpful. There's an image near the bottom comparing a good weld to a bad weld on aluminum.  The good weld is actually the one that isn't smooth.  The type of weld used differs on the type of material used (steel vs aluminum).  A lot of the information on this site talks about coloring, so you can tell if they went too slow and burned the material, but this would often be covered up by the paint on a bike.