The nose especially. But Bobby has blue eyes and his whole brow formation is different. And his Adam's apple is more prominent. Glenn is a bit more padded and less wiry than Bobby. The proof would be to see Glenn's back.
EEEWWWW! I hate dirty fingernails. I am glad I was too taken with Bobby's face and cock to notice those dirty little digits. I volunteer to give him a good scrubbing.
It's not so much the dirty fingernails themselves, more that they represent something that a young skaterdude would have when he arrives home from the skate park, all hot and sweaty and hot to shoot a load.