How can I tell the difference between blood coming from my lungs and airways versus my stomach?  

Blood coughed up (hemoptysis) is usually bright red, frothy, and mixed with mucus, while blood that is vomited (hematemesis) is typically dark (sometimes resembling coffee grounds) and mixed with food. You can often differentiate between them by the associated symptoms: if blood appears when you're coughing, it’s likely from the lungs or airways; if it occurs during vomiting, it’s more likely from the stomach. In either case, prompt investigation is needed.