I found a gm 12 bolt that looks to be off a 70s? el camino i cant tell because its mostly stripped, the only left with the car is the frame and a couple body panel. The rear end is on the ground right below the rear of the el camino. How would i find out if its posi..etc..year...and also how much do these rear ends go for avg in working condition? Thanks

Take it apart is the best way....people have messed with these cars over the years so much, you really dont know what you have.

If you think that it's been un-touched since production, there is a code on the passenger side axle that when de-coded will tell you what Gear, and i believe wether or not its a posi rear.
I may be wrong about the placement of the code, thats where they are placed on an early camaro.

Pop the cover and take a look, they sell for about $200-400 in my area in rebuildable condition when you can find one. I've seen them rebuilt and ready for install with brakes etc. on ebay selling for $1500. The nova and Camaro housings tend to bring more $$