Yes you can tell what animals carry the Jag gene. A jag is a visual het for leucism. They carry one leucistic gene which is recessive (i think or it could be co-dominant), if you can't see jag in them they do not have the 'jag' gene.
Then that would mean that the first one was line bred, because the adults where normal. Or am I wrong?
I'm not trying to prove anything I'm just trying to figure the situation out...
Kane