How to know if it's a clot, or just a tight & sore muscle in calf or behind knee?

Swelling, history: Calf pain/soreness itself fortunately does not raise concern for a blood clot. Significant swelling of 1 leg but not the other especially in the context of recent immobilization (long flight, injury requiring hospitalization/bedrest) or surgery would suggest a risk of a blood clot in the veins of the leg. History of a clotting disorder or cancer also contributes to suspicion.