I am working with a few friends on value studies and pushing our values to extremes. It just occurred to me that many may not know how to discover value in a scene so here is my method.
Because our eyes work like a camera on automatic meter reading, we tend to average value from the full visual field. We should, therefore, start by limiting our visual field to the subject we are trying to draw or paint. This is true even if you are looking at a computer screen or if you are looking at a lit image in a darkened room. My method of limiting the visual field is to forming my hand into a fist while keeping a hole to see through (sort of like we used to do when we were young to imitate a pirate's Monocular). This also acts as a means to frame the image you are studying and help you compose it better. Once you have limited your visual field and composed your image, you will want to squint until the colors of the objects you see fade to gray shadows. This is your value study. Capture it in a small sketch and you will have a much better handle on your subject when painting.