In a previous post, I wrote about the problem with using a red filter as an aid to evaluating value in the landscape. The tool I prefer is the Viewcatcher. A simple piece of grey plastic with a slider, its main purpose is to help select and isolate a scene. But it's got another use, too. There's a hole in it that you can use for judging value.
The Viewcatcher is colored a neutral, mid-value grey. If you're having trouble discerning the value of a particular part of your scene, you can look at it through the hole. If what you see through the hole is darker than the grey of the Viewcatcher, you can call that part "dark." If lighter, then you can call it "light." If it matches the grey value, then you can call it a "mid-value." I know this sounds obvious, but right there you have three values. You only need four to paint a landscape.
I find it particularly useful when I'm trying to decide if some light shape, such as a sunny roof top, is lighter or darker than the sky. Although both will look light in the Viewcatcher, the grey surrounding the hole makes it easier to judge the relative value.
When you use the Viewcatcher, try to keep the side facing you in shadow. That is, don't let sun spill on it. Keeping it in shadow will make it easier to evaluate value.
By the way, my Studio Sale is still going on. Also, I want to remind you about my online video course. I've beefed up the offerings by adding oil and pastel supplements to the basic course.