See a Doctor. A hernia is a hole in the abdominal wall. They can occur anywhere, but commonly in the groin, belly button, and previous incisions. Treatment depends on your symptoms. If they do not have incarcerated (stuck) bowel or pain, then they can be monitored. If they cause problems such as pain, cannot push it back in, etc, then it will need surgical repair.
Multiple ways. Most are clear by physical exam ("turn your head and cough"). Others can be seen on ultrasound or ct scan. Dr. Grover's answer covers the other half very well.
Bulge/Surgery. Abdominal hernias can take many forms yet typically present with a bulge, pain, or both. They usually occur in the groin (inguinal, femoral), belly button (umbilical) or through weak points in the abdominal wall like scars (incisional). Other less common hernias also exist. Treatment is usually tailored to symptoms. But as a rule, non-reducible bulges, vomiting, or pain mandates surgery.
"Turn and Cough" The best test to diagnose a hernia is a careful physical examination by a hernia surgeon. Many hernias are as obvious as the nose on your face, others can "play possum" and require a second exam. On rare occasion, a ct scan can be helpful. Many but not all hernias require surgical treatment. If you think you have a hernia, see a hernia surgeon.
Hernia diagnosis. A hernia can be diagnosed with physical exam, hostory, symptoms and imaging. This is for abdominal wall hernias.