How can I tell if these vintage boardgames are worth anything?
If you look at who wants a game on BoardGameGeek, you can get an idea if there is any demand for the game.  (It's under the statistics section on BoardGameGeek.)   Unfortunately, for any item in a market, it's hard to know if people are actually buying that item.  While I haven't done it for board games, I've followed eBay auctions with active bids to see what they close at.  It can be difficult to see if an item will sell, without trying to sell it.  I'm skeptical of price guides for antiques cause just cause people say something will sell for X, doesn't meany anyone will pay that for it.

You could always donate them to a something like the Salvation Army or Goodwill if you'd just like to have them find new homes.  I'm not sure it costs money to put things up in the BoardGameGeek marketplace, but you could list them on their instead to see if they're interesting to general gamers.