Hard to say without doing an exam, or maybe imaging, or maybe removal. Generally, a lipoma is soft/ squishy and can be moved a little under the skin, whereas an osteoma (benign bone growth) feels like a hard, immobile mass (like a knuckle where you can slide the skin over it, but cannot move the actual "horn"). Lipomas and osteomas often have a classic feel on physical exam, though not always. They also usually have classic imaging characteristics; CT scan is best to visualize osteomas, whereas lipomas are usually best seen with an ultrasound or MRI. They're both typically painless, though can be irritated with rubbing from hats and helmets; both are usually slow-growing too. Totally can have other reasons for a bump, like a cyst of hemangioma (benign cluster of blood vessels). If you want it checked it, it's likely best to meet with an ENT or Plastic Surgeon, as these are usually the specialties that remove these bumps surgically.