Question: How do I know if I’m over- or under-describing a scene?
The trick I use is this: write as if describing a memory. Think about an event that took place in your recent past. Certain details stand out, like the temperature of the air, or the reflective light of the moon off the water, or how a certain object or person felt against your hand.
To go even deeper, maybe the aroma of the barbecue permeated so strongly that you could taste it and the walls of your mouth prickled with saliva. You might not remember what you were wearing, the exact temperature, the wind direction, what the barbecue looked like, or even who was grilling.
Maybe, the moment your tires lost traction on the asphalt, you remember the spike of adrenaline like electricity through each of your veins. You might not remember how the moment of impact jarred your body, or when the glass shattered, or what direction your car skidded in.
I never remember every single detail of a memory, but my brain fills out the rest with unimportant ambiguous shapes. Your reader will as well. It’s important to guide your reader and not control them. If you look at a professional or classical painting, certain things will catch your attention first, as intended by the artist’s composition. The rest of the painting bleeds out from the focal point, leading the eye of the viewer in a dance across the canvas.
The little details, crisp and unchallenged by overt competing description, are the focal points in the pictures that you paint with words. Guide the eyes of your reader in a dance across your world.