To identify mags, just remove the barreled action from the stock. If the mag stays in the stock then its Centerfeed, if it comes out with the action then its a staggerfeed.

Long Actions are simple, regardless of mag style, the screw spacing is 5.062".

Short actions are 4.40" for Centerfeed actions and 4.27" for Staggerfeed actions.

The "J Series" actions are an animal unto themselves. They are a 4.52" spaced and have a Staggerfeed Magazine, but they are longer than a short action and shorter than a long action. They are older and not terribly common, but not rare either.

Hopefully that helps.