Is it allergies or is it a cold? I can never tell, so is it ok to take Claritin (loratadine) whenever my nose runs?

Allergy vs. "colds" Allergic reactions are more likely to involve itching, especially in the eyes. Itchy watery eyes in combination with nasal congestion or "runny" nose is likely to be caused by an allergy. Tracking pollen counts may be useful for observing possible cause and effect relationships with allergy symtoms. If a fever or body aches is present with nasal symptoms think 'not allergy".
Claritin (loratadine) safe. Nasal symptoms with fever is probably a "cold" whereas nasal symptoms with itchy nose and/or eyes are probably due to allergy to pollen, dust, animal dander, etc. Claritin is safe to use regardless of cause but ineffective with a "cold." claritin-d is safe (unless you have high blood pressure or heart disease or other contra-indications listed on product) and will give some relief with a "cold.".