Download and install ImgBurn. www.imgburn.com Start up the program and click on Discovery.
Look at the top of the window and click on Tools-->Drive-->Capabilities.
You will see two lists of different types of disks with little boxes next to them. One list is for writing, the other for reading. If your drive can burn DL media, it will have a check mark in the box next to the DL disks in the Write list.