“Train hard” is useless advice because everyone believes they already do. The useful version is a way to tell, during the set, how close to failure you actually were.
Reps in reserve
At the end of a set, ask how many more reps you could have done with good form. That number is your reps in reserve. Two RIR means you stopped with two left. Zero means you could not have done another. It is a judgment, it is imperfect, and it is still the most practical tool most people can use without a coach watching.
Most productive training sits between three and one reps in reserve. Above four, the set is mostly practice. At zero, every set costs more recovery than the last rep was worth, which is why training to failure on everything works for about three weeks and then stops working.
The tell that does not lie
Rep speed. On a set taken close to failure, the last two or three reps move visibly slower than the first ones, even when you are trying just as hard. If rep ten moves at the same speed as rep one, you had more than a few left regardless of how it felt.
Effort is a feeling. Slowing is a measurement. Trust the second one.
Why people misjudge it
Beginners almost always overestimate how hard a set was, usually by three or four reps, because discomfort is unfamiliar and reads as limit. The correction is not to push harder immediately but to keep doing sets until discomfort stops being surprising, which takes a few months, not a few sessions.
Experienced lifters misjudge in the other direction on movements they enjoy and in the first direction on movements they do not.
Using it
Pick one movement per session and write the RIR next to it. Do that for a month. You are not trying to hit a target; you are trying to find out whether your sense of a hard set matches what your reps actually did. Almost everyone finds a gap, and closing it is worth more than any change of program.