I've heard that it's useful to determine your weakest muscle group and to work more on it. Sometimes it can been clearly seen, but if not, how do I actually do it? Is there some approximate proportions of weights in different exercises to find out which muscle groups is relatively weaker?
I think you're asking a good question but the way muscle weakness and imbalances show up, and the way to fix them, is a typical component of training.
First, make sure you're using a proven strength training program. Not something from a magazine or from some dude's website.
As you start working your way up in weight (keeping an eye on strength standards), you'll most likely start identifying weaknesses. Just as an example, I needed to work on my abs to get through a plateau in my deadlift.
Good strength training programs typically will talk about assistance exercises that will help the primary ones: don't overdo the assistance work and lose focus on your big lifts.
So you're right to concern yourself with imbalances, but the bigger picture is that you need to be on unified program which in turn reduces imbalances and gives you a framework on how to expose and solve them.
This answer solely depends on your overall goal. Which muscles are we talking about, anyway? Usually it is easy enough to tell by looking at yourself which muscles are stronger and weaker. For example, my biceps are about 3 times as big as my triceps, which is unusual because I work them less, but genetics probably determines some of the outcomes as such. That being said, people like me only come once in a Blue Moon(well, except the guys who only do their chest, abs, and biceps for girls)... But if you are a true bodybuilder or weightlifter... Your biceps are probably the weakest muscle group on your body right now, followed by your abs. Your legs will be the strongest and your back will follow after that. However, it is extremely important to keep working your major muscle groups, too, because they will produce the most mass. You should be working your back and legs 3 times a week, but if your biceps are what one would suspect, you can throw those in there, too. Just don't do it for centuries on or else you will end up with imbalance, or having much bigger biceps than triceps such as me.