CBC (rule out anemia)
LFTs
Lipase
Consider acute abdominal series if concern for perforation (>50 years old; concerning abdominal exam)
Consider RUQ US
Consider ECG
Consider troponin
Diagnosis not typically definitively made in ED (requires endoscopy or H. pylori test)
Depending on clinical certainty can consider initial empiric treatment
Peptic ulcer disease
Gastroesophageal reflux disease 
Gastritis
Peptic ulcer disease with or without perforation
Gastroesophageal reflux disease
Lower gastrointestinal bleeding
Pancreatitis
Gallbladder disease 