Lactate (normal lactate highly suggests another diagnosis)
Serum lactate >8 mmol/L has 94% sensitivity
VBG and ABG (narrowing of the venous-arterial PO2 gradient, causes venous hyperoxemia/increased redness -- as does CO poisoning)
Co-oximetry
Chemistry (anion gap acidosis)
RBC or serum<unk>  levels (unlikely to return in time to be clinically useful)
Serum lactate >8 mmol/L has 94% sensitivity
Smell of bitter almonds (only 60-80% of population can detect this)
Severe unexplained metabolic acidosis (lactic)
PO2 of venous blood similar to arterial blood
Normal SpO2 (same as CO poisoning)
Cherry-red skin color is uncommon
Cyanide toxicity
Blister chemical agents 
Chlorate toxicity
Lewisite 
Sulfhemoglobinemia
Sulfur mustard 
Methemoglobinemia
Phosgene oxime 