How can I tell if I have internal stomach bleeding?

Darkened stool. Medication, alcohol, infection, stress, certain food can all cause irritation of the stomach lining. If the bleeding is minor/slow - you may not have much symptoms at all. Patients with ulcers frequently report sharp and severe upper abdominal pain and dark stool (can be bright red if bleeding significantly). Stool check, contrast radiography, or endoscopic exam are helpful for diagnosis.
Doctor's help. It would be difficult on your own without proper testing. Maybe would see vomiting of blood or dark back foul smelling blood in the bowel movement. Otherwise would need blood test to check for anemia, and gastroenterologist for possible endoscopy.