there are several things that can indicate a cold well vs a good weld...
all a cold weld is ..is that the tubing being welded did not get enough heat transfer to properly bond ( fuse) the metal...it is just enough to lay the bead with a weak tack to the tubing...the bead will look ok to the untrained eye.
the dis-oloration of the metal is a Big indicator..i have heard some of the best tig welders in the world refer to the dis-coloration of the metal as a golden trout color..
there is a huge differnec in patch panel welding and chassis welding...
what i would do if i were in your shoes..is find someone who is an accomplished welder and let them show you the difference between a well seated weld and a cold weld...once you see it with the eye you will recognize it...also as i mentioned in my last post a good weld ( seated) with proper heat etc...will have a certian sound over a cold weld,,..you will also be able to see the metals melting and fusing together thru your helmet shield..get some scrap roll bar tubing and practice with it...i assure you that you will see the difference the more you play with it...also remmbre to push your gun.
there has to be a certain transfer of heat from the bead to the metal itself to fuse the metals properly....play with your heat ranges..
if your using a Mig then i am sure you know to stay with Mild Steel.
i would think the COLD WELDS is more than a common problem with DO-IT-YOURSELF projects....Most people are running around with Cold welds all over their car...
you can get lots of info on this but nothing will take the place of getting around someone that can demonstrate for you...and of course Pratice.
check out this site..look at the pictures of the improper fusion of metals.
http://www.weldingengineer.com/Discontinuities_.htm
Good luck,
Brian