If you have a specific hairstyle in mind for a character, you might try asking a good hair stylist what it is called. Hair stylists have to know these terms because they have to create these styles for their clients. You might also look for articles about hairstyles in fashion magazines or books about hairstyles.
Keep in mind that many people do not have such specialized knowledge. For instance, if you are writing from the point of view of a character who is a hair stylist or who is obsessed with fashion, it would make sense for them to be up on all the latest hairstyles and use the correct terminology.
On the other hand, if your main character is a “redneck” male with no interest in fashion, it would seem odd for him to know the correct terms for dozens of hairstyles. He might simply perceive someone’s hair as long or short, straight or curly, black or blonde, etc. Even if he describes someone’s hair in great detail, he probably won’t use the same language a hair stylist would.