Several ways.
First off, go to MPM2008's 12 Gauge Pump Action Shotgun page for some help with the reciever and mag tubes.
Now, a Mossberg 500 has kind of a rectangular ejection port, with a visible extraction lever inside the action. Also, a Mossberg 500 has a flat, non removable magazine tube front. It doesn't have an unscrewable cap like an 870.
An 870 has an oval like ejection port with a small hole punched in the front. The action is other-wise smooth and clear.
If either guns have extended magazine tubes, look at the action.
Also, a Winchester Model 1300 has a rectangular ejection port, but it is more rounded than the Mossberg and no extractor levers are seen.
Finally, a Winchester Model 12 has a very similair look as the Model 1300 except you can see how a 12 has a take down method at the end of the magazine tube and that the reciever and barrel can detach from each other with a twist.
If anyone has corrections for this info or is confused, please add your input.
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