Remington's website has a history section that you can look at.
By the serial number and markings on the barrel.
The value of a 20 gauge Remington Sportsman 58 would actually depend on a number of things. A couple of those things would be the age and condition.
Please check the link for the Manufacture Date codes details on the Remington Society of America.
Should be on the action, Sportsman could be the 48 or 58 semi auto. Sportsman models have little parts left so try not to break any.
There is no model number 3006 or 243 in the Woodmaster line. Check Remington's webiste under the Sportsman's Library.
50-500 USD depending on specifics
100-500 USD
Lookup date code on Remington Society of America here: http://remingtonsociety.com/rsa/questions/barrelcodes
If it's not marked it's a model 12 made in 1914.
Not marked, Model 9
On the bottom of the receiver, ahead of the carrier cutout, or on the left side of the receiver toward the bottom. sales@countrygunsmith.net
You can sell it to a bunch of gang bangers for like $400 and some crack rock.