In doing venous blood gas, what are the markers that tells you a patient is dehydrated and requires bolus of fluid?

Dehydration is often best assessed by a physical examination (blood pressure, pulse, dryness of mucous membranes) & a urine specimen (specific gravity tells a lot about hydration status) . Blood gases are usually done from an artery. Venous blood shows blood urea nitrogen & creatinine levels (kidney function) which can help assess hydration status - those are the most helpful venous blood tests

If that venous blood gas sample was also checked for electrolytes, Na, Ca and K, high Na is due to dehydration.