The first question to ask is: does your character have any? You may laugh at this question (I did), but it’s important to know. Old men aren’t the only people who are bald. Women go bald, as well as young men. Even children can have no hair.
If your character has no hair, why is that? Is it because of age or heredity? Disease? Radiation and/or chemo treatments? Sports? Fad? Showing support for someone who has cancer? Was the character’s head shaved as a punishment or as a result of a dare or a lost bet? How does your character feel about not having hair?
Does your character wear a wig to cover thinning hair? A toupee to cover baldness? A wig to disguise herself? Does he do a comb-over and think that no one notices that he parts his hair just above his left ear?
And if your character does have hair, what color is it? Is it prematurely gray? Is it dyed? Is it an unusual color (for example, natural red-heads are actually rare). A friend of mine’s son is exceptionally tall and has bright red hair. She always told him that he needed to think twice before he did something wrong, because people may not remember his friends doing the same thing, but they would remember him. Does your character like her hair color, or does she not even think about it?
Is it clean or dirty? If dirty, why? Is it because your character is homeless or too poor to buy shampoo on a regular basis? Or is your character too busy, a mom with four young children or an overworked employee? Or maybe your character is absentminded and just forgets? Maybe he is physically or emotionally not able to wash it. Is your character obsessive about washing her hair, even when it doesn’t need it? Is she picky about which shampoo she uses or does she buy the store brand?
Is it thick or thin? Coarse or baby-fine? Straight or curly? Is it hard for him to find someone who can cut his hair so that it behaves and not stick out all over the place? Is it long or short? Is the length his choice or is it forced on him by his job or his school or his parents? I remember my great-grandmother had hair that almost reached her knees. She kept it twisted up in a bun, but she was very proud of her hair. It was thick and wavy, a beautiful black with very little gray, even though she was in her eighties. When she fell and broke her hip, she went to live with my grandmother. It became too hard for my grandmother to have to deal with all that hair, so she cut it off very short. I can still remember my great-grandmother crying over the loss of her hair, her identity.
What about hair elsewhere? Does your character have a beard or mustache? If so, what does it look like? Is it scraggy or trimmed regularly? Does he have a beard to hide a weak chin or to make himself look older? Does he have mustache because his father has one? What if she has a mustache? Is she embarrassed about it or does she even care? Does he have a hairy chest? A hairy back and arms? Does he have to shave twice a day? Or is his body hair very pale or not there? Does it bother him to be so hairy or so hair-less?
How does she wear her hair? Does she enjoy the freedom of a very short haircut, even if it looks mannish? Does she enjoy the variety of hairstyles that long hair gives her? Does she get her hair cut in one style every time, the same style she’s been wearing for the past twenty years? Is she daring with her hairstyles? Does she enjoy shocking people with the latest strange fad?
Hair is very much a part of our identity, how we see ourselves. It’s very important to know how your character feels about the hair on his head and elsewhere.