What Phyr said below seems reasonable, but it's next to impossible to check (because you can't train both GS and skills at the same time).
My answer would be, unless it's clear that a player is not there yet, there is no way to predict if a players is capped just by looking at boxscores and skills alone. The only person who knows is the trainer who doesn't see a pop for a few weeks after he's been expecting one. Also I assume that every player I see on the TL who could be capped is being sold because he is capped. It's worth mentioning that looking at Coach Parrot it seems like it's possible to give training past the cap, however it would be significantly slower.