Other than squinting, I like to use the following tools to ensure that I have nailed the values and drawing in my painting.
A black mirror and a red Compose It Grid.
When double checking my values throughout the painting day I find The Compose It Grid Viewfinder to be very helpful. The red tint allows only red light to be transmitted through the plastic, simplifying what you see. This simplification of color allows one to more easily perceive the value structure. I hold the view finder up to my nose and look through it at my painting and reference to help identify if there are any flaws in my value structure. Of course when you are painting red objects, the red viewfinder is not as effective.
Compose it Grid available at Blick and Amazon.
A black mirror (I found mine on Amazon.com under scrying mirrors.) I use the black mirror mostly to help analyze my drawing in my paintings. With your back to your painting and reference, hold up the mirror so that you can see both. Look back and forth between your reference and your painting while double checking the major angles and shapes. Anytime you can change the way you look at your painting to get a different vantage point it can aid in seeing and correcting mistakes in the drawing. A regular handheld mirror is also useful in analyzing one's drawings and paintings. One can also use the black mirror to double check values.
6" Black (scrying) mirror with a beveled edge from Amazon.