In the video below I show you how I check for crossed hairs in a bow. Just a single crossed hair can have a negative impact on your tone, giving you a gritty and/or metallic sound.
Essentially, the bow is tightened to just above playing tension. A thin straight edge or small ruler (a business card can be used in a pinch if your picking up your rehair from a shop) is inserted into the ribbon of hair at the head and is carefully brought back towards the ferrule. This can be done several times at various points across the width of the ribbon.
Any crossed hairs will be immediately evident as you can hear it binding and can visually see hairs criss-crossed. Note: A little overlap at the ferrule is okay as long as the hair is not crossed. Depending on the dimensions of some bows it can be difficult to avoid.
In the video I demonstrate how to check for crossed hairs and what crossed hairs look like. Additionally, I inspect a bow that I had recently rehaired that is picture perfect!