I have seen many articles telling you what your seams should look like at the proper tension, but all of them have been really vague about how to go about adjusting the tension to get that seam.
First you need some kind of starting point. Older Singer machines were factory set to sew medium weight fabric with number 50 mercerized cotton thread and a number 14 needle with the tension dial set at 3. Nowadays that needs some explanation. As far as I can determine by medium weight fabric they mean something like a man’s dress shirt is made from. For thread I am using Coats & Clark Duly Duty Plus Polyester. I think it is a bit heavier than #50 cotton size wise (I am not real sure of that as I can not find any cotton thread around here to compare it to), but Poly stretches a bit so you usually use a slightly lower tension, because of that it seems to be about right. I also picked up some #12 Schmitz Sharp needles. Anyway I set my machine up so the dial is at 3 with that combination, and it seems to give a good range of setting for other thread and needles.
A side note: I said in the article about Needles, Thread, Fabric and Tension that all of them make a difference. With a bit more experience I now think the most important thing is to match your needle to your fabric. I found that medium heavy Duck breaks #11, and bends number 14 needles, but works fine with #16 needles. At the moment I am using #12 and #16 sharp point needles for most everything.
Now to the matter at hand, how to go about setting your tension when you have no idea what is going to be correct. Start with the dial set at 3 or the center of the dial if you have no idea what the machine is set for. Run a bit of a seam on a piece of dress shirt weight fabric folded double, a couple of inches is enough. I suggest marking one side so you can easily tell which side is the top. Examine the seam.
If there are loops on top reduce your tension two dial marks.
If there are loops on the bottom increase the tension two dial marks
If there are no loops go to the next step
If it looks like sewing on the bottom, and just a straight thread on top, increase your tension one mark.
If it looks like sewing on top, and just a straight thread on bottom reduce your tension one mark.
If it looks like sewing on both sides go to the next step.
Look very carefully at the stitching,
If it looks loose on the bottom increase the tension ½ the amount you changed it the last time.
If it looks loose on the top reduce the tension ½ the amount you changed it the last time.
Repeat until you can not tell any difference in the two threads.
You programmer types will recognize that we and using a binary search to find the correct tension setting. For others what that means is we are moving the setting past what it needs to be and then back half way. We are reducing the error 50% or more each time we do that. Going from loopy to good enough only takes five steps; 2, 1, ½, ¼, 1/8. A couple of steps more and you are moving the dial only a hair and have it about as close at a really well set up machine will do because there is some slop in the train and it will wobble that much. You will also notice that correct tension is only about 1/8 a dial marking from incorrect.
Make notes of what is correct with each needle/thread combination you use, and you will be able to set the dial close next time you use that combination. From there it only takes one or two steps to get it just right.