As far as I know, there is no possible way of telling if they were used in combat. Even the latest made ones, and I don't know when they stopped being made, were used in Vietnam. I've seen them still in use in Afghanistan.
Give up on one that's never seen action and may have been carried by a man (or woman, in Russia's case) that may have died for their country in the end.
What better honor than to keep a hero's weapon in working condition and using it? I'd love for my son to grow up with my issued SAW, but because of all the panty twisters in the government, he'll be lucky to one day own my little pocketknife.
A good honor for a hero's weapon after it's retired is to use it to put some food on your table.