Skulling and examining the bird for fat deposits
The feathers on the head are wet with a drop of water and pushed out of the way.  Now the skull can be examined with a bright light to look for closure of the sutures of the skull.  Juvenile birds do not have closed sutures. 
Holding the bird in this position, the feathers can just be blown up out of the way to reveal the "V" of the wishbone and determine if there is any fat deposited/stored in the area.
The belly of the bird, where the brood patch is found, may also have fat deposits.  But this is a good position to examine the vent for diarrhea.