Find the release date of your DVD. Search online to find out when the DVD was released. For movies, the newer the DVD, the more it will be worth.
Check if a newer version of your DVD exists. For example, the Disney movie "Sword in the Stone" has two releases, a standard release and the Anniversary Edition DVD. The newer Anniversary Edition DVD will be worth more than a standard disc.
Compare it to used DVDs on Amazon. On a regular sale listing, you can look at the "Used Options." Go past the lowest and scroll down until you see a majority of prices around the same range. This is the basic value of your DVD.
Value the condition of your DVD. Take into account such factors as if it is still shrink wrapped, is in perfect condition or if it came with any bonuses like mini-figures or t-shirts. All of these can raise or lower the price.
Include any rare extras that may increase the DVD's value. For example, a wrestling DVD with a wrestler's autograph on it will increase the value. A store exclusive can also help increase the value from a standard edition.
See what others are paying for the DVD on online auction sites. Search the DVD title and instead of seeing current auctions, view the most recently ended auctions. Those prices will indicate a clear value of the DVD you own.