I just recently went to buy a baby Jag.
First I watch them in the tank. Normally I would look for the most aggressive. The ones that chase everyone else around.
But that's not how I chose this time.
They were all under 1", I had the guy pull out 6-8 of them and put them in the little plastic container. While they were in there, all the previously aggressive ones where stressed and scared, but there was one little guy who was in good shape physically, no ripped or clamped fins, no ich, no coudy eyes, who was trying to eat some of the debris in the container while all the bigger ones were just huddled in the corner.
So I got the little guy. I figure, he's not even scared.
I got him home and he's never been shy. He comes out to see what's up when I look in the tank and eats very well.