Q: Assuming patient has normal renal function, what is a laboratory way to determine that GI bleed is likely to be upper?

Answer: 

If the ratio of blood urea nitrogen to creatinine is greater than 30 the source is more likely from the upper GI tract. 

Reference:

Srygley FD, Gerardo CJ, Tran T, Fisher DA (March 2012). "Does this patient have a severe upper gastrointestinal bleed?". JAMA 307 (10): 1072–9