Experienced, body-aware lifters develop a good sense for this. Inexperienced lifters, though… well, it gets tricky. In good studies, the workouts are supervised and every parameter is enforced. Out in the real world though, you see a lot of what’s called fake failure.
Sometimes I just don’t think the new lifter is trying hard. Other times, I think the new lifter thinks he’s trying hard. Get a good coach in front of him though and suddenly he can get several more reps, with good form.
I think it was Thibaudeau who said that some people think failure means stopping when the sets gets hard, but he said something like “the set begins when it gets hard.” I think this was when he was discussing the effective reps theory. Maybe HERE.
Even some experienced lifters have issues with this. For example, someone can say, “I always train to failure and it works better for me!” But they don’t always train to failure; they just think they do.
After 25+ years in this biz, I’ve seen it often, even with coaches. Some say, “This is the best way to do it!” then demonstrate (in person or on video) and you know what? They’re NOT doing what they said they were doing. Heck, I’ve had to rewrite their entire articles to make their text match their videos. And I’m not really slamming them; they’re big fit dudes. But it kinda goes to show that these little details sometimes don’t matter.
One example: The coach says, “Always use a 4-second eccentric! Always!” Then the same coach sends me videos for the article where he (wait for it) never takes 4 whole seconds to lower the weight. 
I change the text to: “Lower under control for 2-3 seconds or so.”