The diagnosis of breaks in slumps is a little more difficult than in full fuses. In full fuses sharp edges to the break indicate the fracture occurred on the way down in temperature and rounded edges indicate the break happened on the way up.
But the temperatures in slumping are not high enough (in normal slumping procedures) to round no matter when the break occurred. If the break occurred on the way up, it will remain sharp, just as it would if it broke on the way down. There needs to be another way to diagnose when it broke.
The key is to try to fit the pieces back together.
If the break occurred on the way up, the pieces will separate to some degree – depending on the force of the stresses. After this break the glass will slump into the mould from their separate places. When cooled and you attempt to put them together they will not fit perfectly.
If the break occurred on the way down, the whole piece had already slumped as one. The broken pieces will fit together, as the slump broke apart after the form was achieved.