chevy straight 6s
 

how do i tell the differance between say a 235 and a 250 inline six chevy motor

While I have no direct experience with those engines, in general, engine manufacturers will cast engine numbers somewhere on the block that should identify the engine. You can sometimes find that sort of identification info in maintainence books for vehicles that had both engines in the total production run.

The emissions sticker under the hood will tell you the displacement in liters.

that was the first thing i looked for but this is a 71 pickup and its mostly roten but runs cso there is no emissions sticker

The 230s and 250s in my Novas had the engine code stamped on the block on the distributor boss. They where covered in years of goop, but still there when I cleaned it up with a wire brush. Look there, then find a Chiltons for the truck or search on the web with google. I have had some luck finding the engine suffix codes with google before. The code is actually preferred over a casting number because a casting number a lot of times was used for many different applications. The suffix code will tell you exactly where the engine came from. You should get the original vehicle type it was installed in, the displacement, and the tranny that went with it.