If it's a fixed choke barrel, it will be stamped on the barrel and be on the left hand side around 10 o'clock looking at the action end on the Remington 870.

If it has screw in chokes, many after market chokes are available for distance shooting. This one looks interesting although I don't have any experience with it!

Carlson's Coyote Choke Tubes

I've never needed anything more than full choke myself, I choose my shot size for gaing more range but sometimes on game you want to choose the next size smaller, that way you can put more shot on the target.

It all depends on what you're trying to shoot at a given distance. Don't expect anymore than 100yds with (buck, not bird) shot and that's pushing it