When I go to the boneyard, how will I know a 3.42 Posi when i see one? Is there any codes I should know or anying involving the RPO?

by pulling the cover and looking

I'm pretty stupid when it comes to rears. How would i not the difference between a posi and an open?

do a search on hwre for posi and you'll find some pics, print them and take them along if you have to.

If you're looking for stock 3.42 rearends with posi, then you'll need to find a rear from a 87-92 LB9 T5 car with the G92 RPO. You can also find them in 85-86 LB9 700R4 cars with the G92 RPO. So I guess when you're at the boneyard, just make sure the 8th digit of the VIN is 'F', the car has the transmission matching the years I posted above, and the RPO 'G92' is located in the glove box (either in the trunk, or in the center console). if so, grab that rearend man!!

only problem with crazy's method is it only tells you what the car left the factory with, not what it has in it now. finding a 3rd gen f body in a junk yard will be a trick in it's self.

oh yeah forgot about that part

Take a look at this pic. This is a GM 4th gen posi. Any 6spd 4th gen car will have this rear.

Plenty of both junkyards and third gens out here in Suffolk, VA.
O'Malley's has 6 third gens at least, and about 5 fourth genners.

To find out if it's a posi, jack it up and turn one wheel. If the other turns the same way, posi. If it turns the opposite way, peg leg.

while it's jacked up, support the axle, and mark the wheel and driveshaft. Spin the wheel once, and count the driveshaft revolutions, that'll be your ratio.

Wrong!.....my new SLP Posi carrier spun the opposite way when I put it in. So your best bet is to pull the cover and see!