How do you know you have blood clot in your legs?

Clot in the leg/DVT. Deep venous thrombosis can be suspected by asymmetric swelling of legs, tenderness in the calf but confirmed by ultrasound of the extremity concerned.
Look at the leg. Usually presents as either painless or painful new asymmetric swelling of a lower extremity. Sometimes first sign of blood clot that was in leg is blood clot in lung. That shows up with shortness of breath, rapid heart rate and chest pain. Either of these may be confirmed by your doctor with blood tests, doppler studies of your extremities or ct.