I think no one tells them the shorts are threadbare, since it's not something noticeable until the shorts are on and the rider is leaning forward. I was riding with my roommate a few weeks ago, sitting on his wheel, and when the sun hit his back I could see everything. He threw the shorts away right after the ride.

Now I'm worried my shorts are threadbare and no one's bothered to tell me...

So how do you tell someone their butt is showing if you don't know them well?