Testing at GC is quick-&-dirty: Have two plugged into the board, hold one in either hand and sing scales back and forth between the two while listening in the headphones for sound and trouble. Some Oktavas clip at relatively low input levels due to dirty boards. I don't understand quite why this happens, but two engineers I know of independently gave that explanation and I did encounter
MK-219s that did precisely that, for whatever reason.
When you decide which one of the two is better, pull out the bad one and replace it with another untested one and repeat the above, just like you were doing the lens test to get new glasses.
Eventually you'll separate the better ones from the obviously bad this way. Don't forget to work the switches, too. Believe it or not, the switches and the roll-off circuits are sometimes bad on these.
QC with Oktavas is such that unless you're absolutely deaf, you _will_ notice an audible difference in MK-219s, even at this crude level of comparison and with whatever headphones they have lying out on their boards. In my local QC their recording-gear studio is fairly well acoustically isolated from the shredders in the guitar department. I also did this at 1100 on a weekday, right when they opened, which was a good call considering it was still quiet in the store and they had time to accomodate me in this.
I went through eight and bought the two that seemed best and definitely rejected a couple of bad ones. This isn't some serious lab test for guys in white coats, but I think it will determine which are the better ones out of a batch, which is ultimately all you can do. I doubt that my selected MK-219 are materially better or worse than Johnson's selected MC-219s - it's just that Johnson catches the bad ones and repairs them or sends them back and doesn't sell them if they're not right. GC just moves whatever stock ASM sends them by the palletload.
Considering that 012s and 319s are $200 (or more) at GC, I doubt that I'd buy them there, particularly in that the 012 is particularly noted for troublesome circuitry. The price differential between Johnson and GC is small enough on these models that it might be wiser to pay a bit more and get the royal treatment from Johnson.
I doubt I'd ever buy a 319 at twice the price of a 219 when it has the same internals, come to that.