As to "good" joints (lot of experience soldering all sorts of stuff
here), I can tell a good joint generally just by inspection. This
applies to electronics as well as pipe, etc.

You should see a meniscus of solder evenly flowing between the two
items. There should be no "bare" spots, or clear inclusions of crud,
excess solder, blogs, etc. Look at the joint with a magnifying glass,
if possible. If the joint is in a hard to get at location, then
observing the back side can often be done with a good flashlight and
inspection mirror.

I've found, when teaching people to solder, that they should PRACTICE
on some scrap stuff first to get good technique. In dealing with
plumbing, most of the problems are due to much too much heat. You just
want enough heat so that the solder just flows easily. Dirt is your
enemy. Emery paper, wire brushes, and steel wool are your best
friends. Flux simply serves to remove final bit of surface oxidation.

I really like rosin core solder, even for plumbing. The new lead free
solders work at a slightly higher temperature, and in my experience
are harder to get good joints with, especially if you have grown up on
lead bearing solders. I am reluctant to use acid core solder, as the
residue is hydroscopic, and corrosive, and will come back to haunt you
later.

Also, I find it useful to "pre fabricate" as much stuff on the bench,
then finish up with maybe two joints to be soldered in the final work.

Also, in working with OLD plumbing, you have to deal with corrosion.
Sometimes I've used compression fittings instead of soldering. Of
course, use TWO wrenches to tighten them up so as not to twist the
pipe.

Hope this helps..