Guys overthink things that should be common sense. You either have a fixed workout plan or you have a rough one and go on feel. You complete that workout and that's it. If you realize after a few weeks it's too much volume/frequency you change things so you are able to recover workout to workout. If you follow a rough plan and train instinctively then you should simply know when to stop unless you are a complete newbie to the gym. If you aren't feeling it maybe cut it short and if you feel like doing a little extra go for it.
Just go home when you feel like going home and carry on doing that and if things need to change over the weeks you adjust and go from there. Yes some over train and they need to lower volume/frequency but for the vast majority it's usually because their diets and sleep etc are all over the place. If you have things in place doing an extra 5 sets is nothing major. On the flip side some are addicted to the gym and that skews things and they will always do too much but even for many of those guys it's not the end of the world if everything else is in place and it's not like they still won't be able to look great they will just go through life feeling tired