How to differentiate pulmonary embolism from pneumonia on initial presentation?

Patient is a 45-year-old female presenting with sudden onset pleuritic chest pain, tachycardia, and mild hypoxia. She recently returned from a long-haul flight. CXR is essentially normal. How do we clinically distinguish PE from pneumonia, and what is the role of Wells score, D-dimer, and CT-PA in this workup?