This is a piece of the puzzle that most people think they have, but really don't. It's true in the gym and it's true in the rest of your life.
No one will ever tell you they don't work hard. They'll never tell you that they're lazy in the gym. In their minds, they're gym warriors. Well, in all the gyms I've trained at or worked at, I could say that on average only around 10% of the lifters really work at a level that I consider sufficient to elicit maximum gains. The rest of them just think they're working hard.
Quite simply, nobody works as hard as they think they do. Even those who actually work hard can improve somewhere. Working hard isn't about grunting and growling, nor is it about dropping your dumbbells on the floor after your sets so you can look hardcore.
Working hard, with maximal effort, is about one thing and one thing only: mental focus.
When you set foot in the gym, you must enter the zone: only think about training and focusing on getting the most out of every single muscle contraction. Don't waste your time between sets and don't engage in any conversation. (I can't recall a time in the past three years where I said more than one sentence at a time during a workout.)
Don't think about anything else other than your workout. Don't allow pain to stop a set if you still have something in the tank.
Remember, the difference between the champ and the chump isn't what he does, it's how he does it.