How do I know if a liver hemangioma is a problem?

Depends on size. Most liver hemangioma are small, usually less than 1-2 cm, and are found incidentally during an ultrasound or ct exam. Unless they are large and replace a large segment of the liver, they are completely benign and are asymptomatic.
Usually asymptomatic. Liver hemangiomas are disorganized collections of blood vessels within the liver. They usually do not cause symptoms unless they get really large. They don't really have an appreciable risk of converting into cancer. Typically they are just observed unless they are massive or symptomatic. If it meets classic criteria for a hemangioma, you don't even really need to get repeat scans.