Please check the link for the Manufacture Date codes details on the Remington Society of America.
Lookup date code on Remington Society of America here: http://remingtonsociety.com/rsa/questions/barrelcodes
By the serial number and markings on the barrel.
Remington's website has a history section that you can look at.
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.
1920
1914
1913
1920
1919
late 1919
1920
1915