I've actually been basing it off of if the aura is on them. i noticed if the aura isn't there, i can usually feed/play with them once
Interesting observation, my pet did not have the aura this afternoon and it allowed me to feed him. The aura is back now - I think the aura represents when your pet's hunger and happiness stats are maxed out.
This thread: https://www.sensipets.com/forums/bugs-and-errors/appetite+changes-8ebd954a2a95669/1 discusses aspects of pets' hunger and happiness. For the time being, it does seem like pets' hunger or happiness goes down slightly by each (judging by my own pet?).
But I ain't got a hammer / And I ain't got a pencil / And I ain't got a lasso / So I'm doing it the hard way / Like a post post-modern man