The title should actually be "what to watch AND LISTEN for in your yard", as young jays become extremely vocal while calling for attention from their caretakers.
Young Blue Jays are often about the size of the adults by the time they enter your yard. At first, they'll hang around in trees fairly quietly, at least until an adult approaches. Then, like a needy child, they'll turn on the show with a big wing quiver and a desperate, screechy whine for food.
As days go on, the jay nursery may get bigger as more young jays from surrounding areas fledge. Tending to the needy young are not only the parents, but perhaps the unpaired, previous year's birds as well.
When a raucous group of jays descends into your yard, it makes for quite a commotion. Be forewarned, this din can go on for quite some time - you may still see young calling for food well into August! The young will continue to look for an easy handout even when totally capable of getting food themselves.
Early on, look for some of the telltale features of young jays: fleshy corners to their beaks, a shorter tail, or perhaps some feathers that haven't quite emerged from their sheaths. But right now, you could know with your eyes closed when young jays are in your yard - just listen!