How to tell if it is deep vein thrombosis or calf muscular pain and tendonitis?

That is a good: Question as sometimes it is clinically difficult for us too the physicians. To be sure one can do a blood test called d-dimer test but the best and with immediate results is an ultrasound duplex test. That will immediately show if clot is there or not. Trauma can cause clots and muscle pain. So, for a sudden calf pain one has to see a doctor asap as a clot may need specific attention.

Ultrasound of veins: It is sometimes very difficult to distinguish between DVT and calf muscular pain simply by physical findings or history. The definitive diagnostic test is a venous duplex ultrasound which will show the veins behind the knee (popliteal) to see if there is a clot.