Its honestly hard to truly tell but you can some sense it from body language and unless you're in the locker room with them you wont truly really know. Sometimes its not even done consciously, its akin to a team with a big lead letting up, if you ask the players they feel and think they are still playing hard and the other team is just rallying; but from home you can see something is not quite the same.
We also have to remember these are kids we are talking about; physical specimen but they are not all fully mentally developed thats why a dropped pass, a missed block, an int etc can totally take a player out of the game. Now think of a kid who's had miscues 2 or 3 games straight. its easy to say give him a pep talk but in reality its different. Think of that and think of Stacy Coley, i dont know how strong of a mindset he has but imagine 19y/o with NFL dreams (when he signed he famously said "the school i will be attending for the next 3 years..."). He starts the season, first game 3 catches and then no catches until the 4th game, it adds up. Im not saying he quit, but it starts to get on you.