What Spacecoast said is really worth listening to. I would add only two comments: even the "flashlight" test doesn't always work because it is sometimes hard to tell visually just how much powder is in a case. I have come to using either a dowel or brass rod marked from a properly loaded case, dropped into each case in a lot to be sure there is no squib.
My second comment is that I will not use the reloaded ammo from a gun show. If I can make a mistake myself, how can I be sure that someone else hasn't also made the mistake.