From the date code stamped on the barrel. Look for the manufactured date link on the Remington Society of America for details.
The barrel date code is the primary method of dating a Remington. Of course, if the barrel is not the original barrel for the gun that complicates things. The Remington factory could probably date it by serial number. sales@countrygunsmith.net
Look at the date code stamp on the barrel, see related link.
Do a search on barrel codes, I believe that will give you an answer
They have date code stamps on the barrel which can be looked up on the manufacture date link on the Remington Society of America. --- Update: It's important to note that the barrel on the shotgun MAY not be the barrel that was originally on the shotgun at purchase. Check the serial number with Remington.
Go to the Remington Society of America and click the Manufactured link, it has the location and barrel codes for month/year of manufacture.
Remington Society barrel code lookup: http://remingtonsociety.com/rsa/questions/barrelcodes
They are date code stamped on the barrel on left side in front of receiver. A barrel location diagram and the date code table (month/year) is available on the Remington Society of America.
Check the date code stamp on the left side of the barrel you can get the details and look it up on the Remington Society of America under the manufactured date link.
100-350 USD
Check the Remington Society of America, date barrel codes and location. http://remingtonsociety.com/rsa/questions/barrelcodes
To determine the exact value of a Remington Wingmaster 20 gauge model 870, a few different things would need to be taken into consideration. Some of these things would be the age and condition.
Contact Remington Customer Service through their website.