I own a used bookstore and may be able to shed a little light on this.
Nat Geos are worth money (the older the better) to collectors, but only if they retain the original maps.
I'm assuming you're talking about Reader's Digest magazines as opposed to Reader's Digest condensed books. The magazines have no saleable value - please recycle. The Condensed books also have very little value - I usually put them on my Free Rack when they come in (except for Biographies), but if you donate those to Goodwill and itemize your taxes, it's my understanding you can take $1 per (hardback) book.
Many old magazines do have genuine value, often based on their covers or the time period, but they need to be in pristine condition.
Here are some easy places to find market values on books and other publications typically found in situations like yours (these are current asking prices, not bids on auction sites):
abebooks.com (handles many older items)
half.com (the bookselling end of eBay)
amazon.com (used book sales)
The last two generally have the lowest prices on modern publications and those printed in the thousands/millions.
Hope this helps!