I do not think that complete breakage is the object.  With bending or folding over you can see the root and tell if there is fusion along the entire surface of the pieces.  The root of the weld should fuse the entire  square edge of a fillet on the vertical member. 

My interpretation is that there should not be any edge of the plate visible and that the weld should break without obvious slag or other discontinuities (within spec limits)  I test fillets for practice with my students and look for incomplete fusion along the fusion lines and especially the starts and stops which I require at the joint center. 

Many do not fall apart but good visiblity to the weld root is opend and a lot can be learned about the penetration and technique.  Generally I like to see the weld seperate down the middle of the weld without any fusion discontinuities along the fusion edges.  Some may correct me on this I am sure but if the weld seperates at the toes it is probably lack of fusion.  Of course this is with filler metals matching or undermatching the base metal.