One major thing I look at is coat quality. A dog not getting adequate nutrition, regardless of their body weight, is going to have a dry crappy coat, dry skin, etc. An supremely athletic lean dog that is well conditioned and fed well will have a nice glossy coat, like the dogs in pops' picture.
Right now I can feel Strider's every rib. He goes on 10-12 mile runs every day, with speeds varying from 12-35+ MPH, plus free play time. Part of the problem I think is his kibble though. He refuses to eat more than 3 cups of it a day, and has lost some weight on it and his coat feels dry and crummy so we are switching him to TOTW. He's on satin balls now too. I can feel the muscles in his shoulders starting to bulk up a bit finally.
Nice dogs by the way! Some day, it is my dream to put together a team of boar hunting dogs. Here in Washington it is free and open to hunt european wild boar any time, no license needed at all. The state wants them exterminated, but they just keep spreading.