Looking at your crumb structures, I don't think you need much help from me or anyone else.
Nonetheless, this is one of those cases where it is far easier to show someone than to describe the matter using words. However, one simple test you can use is called the windowpane, or gluten window, test. It's been so long since I have used it, that I completely forgot about it. But I think it is a good test until you get to the point where you no longer need to rely on it. Itís also a good crosscheck on the condition of your dough.
To perform the test, once all the dough ingredients, including the oil if you use it, have been kneaded together and the dough starts to take on a smooth, kneaded appearance, you take a piece of dough about the size of a walnut and flatten it in all directions so that is basically a thin disk. You then stretch it outwardly in all directions by gently pulling the dough outwardly toward the perimeter, grasping as much of the dough at the perimeter as you can so that the dough stretches in all directions. If the dough has been properly kneaded, you should be able to see light through it and there should be no tears in the dough as you stretch it out.
If there are tears in the dough, that is usually a sign that more kneading is needed. If it's also wet and sticky, you will most likely also need more flour. If you want to see an example of the latter condition, take a look at Reply # 21 and Reply # 25 at http://www.pizzamaking.com/forum/index.php/topic,1698.20.html
. You continue the kneading (adding more flour if necessary) and repeat the test until the dough passes the windowpane test. Sometimes I do the additional kneading by hand because I like the control that hand kneading provides over the kneading process. I think my hands also do a better job of kneading than my machine. For an example of a properly developed gluten window, you may want to take a look at http://www.redstaryeast.com/baking/lessons_twoc.html
What I have discovered is that when the dough can pass the windowpane test, it will usually have all the desired characteristics you want in the dough--at least the ones I look for. The dough should be smooth and elastic (some recipe writers also use the term "shiny" but that's not always my experience) without any tears on the outer surface when you squeeze the dough to put the outer skin under tension. I prefer that the dough be a bit tacky, but not sticky or wet. That's a threshold test, and it is not fatal if the dough otherwise feels smooth and elastic and doesn't have any tears in the outer surface.
You may want to also keep in mind that not all doughs are amenable to the windowpane test. Weak flours that are low in protein and gluten, like some 00 flours, and whole-wheat doughs or doughs including cornmeal, do not do well with the test. It is best for white glutinous flours, like all-purpose and above.
Since I don't intentionally allow my doughs to get to overknead territory, I don't have many signposts to give you at the dough level other than to say that the dough is likely to get tough, rubbery and dry the longer you knead it. If the dough is allowed to go beyond overkneading, the gluten network will be severely damaged. If the temperature of the dough gets above about 140 degrees F, the yeast will die and the dough will become putty-like and lifeless--and will be completely unusable.
EDIT (4/29/15): For Wayback Machine version of the Red Star link, see http://web.archive.org/web/20040103040948/http://redstaryeast.com/baking/lessons_twoc.html