Use the VideoReader class, and get() the BitsPerPixel property and possibly also the VideoFormat property. Those will tell you the number of bits per pixel, which is the same as the number of user defined bits.
To determine the unused code words, you would need to read the entire video and record all of the pixel values that appear. Any pixel value that never appears anywhere in the video is unused.
Your terminology suggests that you are working with steganography or watermarking. If so then what you are missing is that "user defined bits" and "unused code words" are not properties of videos, they are properties related to the interpretation of images and videos. People doing steganography or watermarking usually chose the user-defined bits such that there is "not much" visual change, but as far as the person receiving the video is concerned, the video has always been exactly what they receive.
If you are trying to detect whether a particular video has a steganographic embedded message or a watermark of some kind, then you need to study research in the area: if there was an easy way to determine it then the steganography would have been insufficiently hidden.
