If the two were placed beside each other, how do I tell magnum action bar from a field model one

The action bars are identical. The action sleeve is heavier on the magnum model with less metal machined away. There are pics in the gallery on ShotgunWorld.
The ONLY differences between the magnum and standard models are the barrels - one or two gas ports - and the action sleeve, and a recoil pad on the Magnum.