In a lot of my classes and workshops, I talk about value finders. Sometimes I ask students to curl up their hands like a telescope and look through the little hole that's formed at the end of the "tunnel." This works well when you out-of-doors. I can't tell you how many times, I've used this simple method to get my values correct.
Another value finder is simply made by punching a small hole into some paper. Like so...
But what's the whole point of this value finder stuff? I'm glad you asked. :) Let's look at the following photo of an orange. Notice there are various tones or values all over the orange. It's fairly easy to see that there is a highlight facing us and shadow underneath the orange. But what about all those subtleties we may overlook? Well, that's where the value finder comes in. You can move it around the orange and "see" all the values.
Here's an example. By using several value finders (since I can't move it around), you are able to zone into how many different tones you see.
What's nice about value finders is that because you wipe out all the extra information with either your fingers or surrounding paper, you're able to focus in on the real tones.